how to define a comment syntax for files having a specific extension?

["gento" <gento_distefano@hotmail.com>]

Hi All, I'm using
"GNU Emacs 21.2.1 (i686-pc-cygwin, X toolkit) of 2004-03-22 on cm-
test"
and working on journal text files *.jou for which the commented lines
must start with the character "/".

While editing one of such files I need to run
M-: (setq comment-start "/")
in order to comment regions with the sequence C-c;

I'd like to have emacs recognize the "*.jou" files and
apply the corresponding comment syntax automatically,
sparing me from having to type
M-: (setq comment-start "/")
each time I start working on a *.jou file.

I appreciate any help