diff -r afe6c433867e tex.el
--- a/tex.el	Thu Nov 30 23:21:22 2017 +0100
+++ b/tex.el	Sun Dec 03 15:21:38 2017 +0900
@@ -2696,8 +2696,26 @@
 	  (setq answers (cons entry answers))))
     answers))
 
+(defun TeX-kpathsea-detect-path-delimiter ()
+  "Auto detect the path delimiter for kpsewhich command.
+Usually return \":\" or \";\".  If auto detect fails for some reason,
+return nil."
+  (let ((res (ignore-errors
+	       (with-output-to-string
+		 (call-process "kpsewhich" nil
+			       (list standard-output nil) nil
+			       "--expand-path" "{.,..}")))))
+    ;; Kpsewhich expands "{.,..}" to ".:SOMEDIR" or ".;SOMEDIR"
+    ;; depending on its environment.
+    (and (stringp res)
+	 ;; Check whether ; is contained.  This should work even if
+	 ;; some implemantation of kpsewhich considers it sane to
+	 ;; insert drive letters or directory separators or whatever
+	 ;; else to the current directory.
+	 (if (string-match ";" res) ";" ":"))))
+
 (defcustom TeX-kpathsea-path-delimiter
-  (if (memq system-type '(windows-nt cygwin)) t path-separator)
+  (TeX-kpathsea-detect-path-delimiter)
   "Path delimiter for kpathsea output.
 t means autodetect, nil means kpathsea is disabled."
   :group 'TeX-file
@@ -2716,17 +2734,15 @@
   ;; FIXME: The GNU convention only uses "path" to mean "list of directories"
   ;; and uses "filename" for the name of a file even if it contains possibly
   ;; several elements separated by "/".
+  (if (eq TeX-kpathsea-path-delimiter t)
+      (setq TeX-kpathsea-path-delimiter
+	    (TeX-kpathsea-detect-path-delimiter)))
   (when TeX-kpathsea-path-delimiter
     (let* ((exit-status 1)
 	   (args `(,@(if program `("--progname" ,program))
 		   "--expand-path"
-		   ;; It seems that kpsewhich accepts semicolon as
-		   ;; path delimiter even on non-w32 platform and
-		   ;; converts it to colon if necessary.
-		   ;; Quote from texmf.cnf of TeXLive 2017:
-		   ;; "In this file, either ; or : can be used to
-		   ;; separate path components."
-		   ,(mapconcat #'identity vars ";")))
+		   ,(mapconcat #'identity vars
+			       TeX-kpathsea-path-delimiter)))
 	   (path-list (ignore-errors
 			(with-output-to-string
 			  (setq exit-status
@@ -2737,34 +2753,8 @@
       (if (not (zerop exit-status))
 	  ;; kpsewhich is not available.  Disable subsequent usage.
 	  (setq TeX-kpathsea-path-delimiter nil)
-	(let* ((delim
-		(cond
-		 ;; If `TeX-kpathsea-path-delimiter' isn't t, use it
-		 ;; for path delimiter.
-		 ((not (eq TeX-kpathsea-path-delimiter t))
-		  TeX-kpathsea-path-delimiter)
-		 ;; Otherwise, if the output begins with DOS drive
-		 ;; letter or contains semicolon, we can set
-		 ;; `TeX-kpathsea-path-delimiter' to ";" and use it
-		 ;; for path delimiter.
-		 ((string-match "^[A-Za-z]:\\|;" path-list)
-		  (setq TeX-kpathsea-path-delimiter ";"))
-		 ;; Otherwise, if the output contains colon, we can set
-		 ;; `TeX-kpathsea-path-delimiter' to ":" and use it
-		 ;; for path delimiter.
-		 ((string-match ":" path-list)
-		  (setq TeX-kpathsea-path-delimiter ":"))
-		 ;; Otherwise, the output of this particular run
-		 ;; happens to contain just one component (without any
-		 ;; delimiter) of relative path (without DOS drive
-		 ;; letter).
-		 ;; It is safe to use ":" as delimiter for this case,
-		 ;; but we cannot determine the value of
-		 ;; `TeX-kpathsea-path-delimiter' now.
-		 (t
-		  ":")))
-	       (separators (format "[\n\r%s]" delim))
-	       path input-dir-list)
+	(let ((separators (format "[\n\r%s]" TeX-kpathsea-path-delimiter))
+	      path input-dir-list)
 	  (dolist (item (condition-case nil
 			    (split-string path-list separators t)
 			  ;; COMPATIBILITY for XEmacs <= 21.4.15