example.tex

\linespace{\section{見出し}} % 自動で何行取るのか計算される

\linespace[3]{\section{見出し}} % 何行取るか手動で入れる

linespace.tex

\makeatletter

\def\linespace{%
  \@ifnextchar[\@linespace\@linespace@auto}
\newcount\c@linespace
\long\def\@linespace@auto#1{%
  \c@linespace = 1
  \setbox\@tempboxa\vbox{#1}%
  \setlength\@tempdima{\ht\@tempboxa}%
  \addtolength\@tempdima{\dp\@tempboxa}%
  \def\@rec{%
    \setlength\@tempdimb\Cvs
    \multiply\@tempdimb\c@linespace

    \ifdim \@tempdimb>\@tempdima
      \def\@k{\@linespace[\c@linespace]{\box\@tempboxa}}%
    \else
      \advance\c@linespace1
      \def\@k{\@rec}%
    \fi
    \@k}%
  \@rec
}

\long\def\@linespace[#1]#2{%
  \par
  \setlength\@tempdima\Cvs
  \multiply\@tempdima#1
  \advance\@tempdima-\Cvs
  \advance\@tempdima-\Cht
  \advance\@tempdima\Cdp
  \setbox\z@\vbox{#2}%
  \advance\@tempdima-\ht\z@
  \advance\@tempdima-\dp\z@
  \vspace{\ht\z@}%
  \vspace{\dp\z@}%
  \allowbreak
  \vspace{-\ht\z@}%
  \vspace{-\dp\z@}%
  \vtop to\z@{%
    \vskip.5\@tempdima
    \box\z@\vss}\quad
  \setlength\@tempdima\Cvs
  \multiply\@tempdima#1
  \advance\@tempdima-2\Cvs
  \vspace\@tempdima
  \par\nobreak}

\makeatother