% \chapter を変更(book.cls からコピー)
\def\@chapter[#1]#2{\ifnum \c@secnumdepth >\m@ne
\if@mainmatter
\refstepcounter{chapter}%
\typeout{\@chapapp\space\thechapter.}%
\if@appendix\def\tocloa{loa}\else\def\tocloa{toc}\fi% (*)
\addcontentsline{\tocloa}{chapter}% % (*)
{\protect\numberline{%
\if@appendix App\else Chap\fi
~\thechapter}#1}%
あいうえお
かきくけこ