In order to avoid this problem, I have just installed the following
change:

    2004-10-07  TSUCHIYA Masatoshi  <tsuchiya@namazu.org>

        * gnus-art.el (gnus-mime-display-single): Call `mm-display-inline'
        instead of calling `mm-insert-inline', to decode text/* parts
        before displaying them.

I, however, worry whether this change is appropriate or not, because
there are several choices to fix this problem.  The first choice is my
proposal.  The second choice is to call `mm-inline-text' instead of
calling `mm-insert-inline'.