Runaway argument? means a command’s argument is missing its closing }, so TeX keeps swallowing text looking for it — usually ending in ! Paragraph ended before \cmd was complete or ! File ended while scanning use of \cmd. It is almost always an unbalanced brace.
What is happening
When TeX reads \command{..., it waits for the matching }. If the closing } never comes, it keeps swallowing text. The error prints the runaway text (what it swallowed), and its start points at the unclosed {. A blank line (paragraph break) inside a “short” argument (one that disallows paragraphs) gives Paragraph ended before ... was complete; if } never arrives, File ended while scanning use of ....
Causes
- A
{whose closing}is missing (most common). - A blank line inside an argument that disallows paragraph breaks (e.g. a blank line inside
\section{...}). - No
}after\command{....
The fix
Balance the braces — the start of the runaway text points near the unclosed {. Find it fast with your editor’s bracket matching. Remove any blank line that slipped into a short argument. If an argument legitimately needs paragraphs (blank lines), the macro must be defined \long (most LaTeX commands that take such content already are).
% NG: } を閉じ忘れ / missing closing }
\textbf{重要な点 …（ここで閉じ忘れ）
次の段落。
% OK
\textbf{重要な点}
次の段落。