最中に|さいちゅうに

Means 'right in the middle of' or 'at the height of' an ongoing action or event. The main clause usually describes something sudden or unexpected that happens at that moment and interrupts it.

Neutral in register and common in both speech and writing. It is typically used to report an interruption or surprising event, often an unwelcome one, that occurs while an activity is fully under way. It stresses that the activity was at its height, which makes it more vivid than a plain time expression like 間に.

• ところに / ところへN2「ところに／ところへ」 can also attach to 辞書形, た形, and 形容詞, and can indicate that something happens right when you are about to do something or have just done it; its second clause is often an outside event such as a visit or a phone call. When limited to being in the middle of an action, it is often interchangeable with 「最中に」.
• 間にN4「間に」 simply and objectively frames a period of time — something may happen during it, or you may deliberately finish something before a deadline — and it does not emphasize being suddenly interrupted. Its attachment is also broader: it can take 辞書形, ない形, and 形容詞, which 「最中に」 cannot.
• 途中に / 途中でN3「途中に／途中で」 focuses on the midpoint of a process and can also refer to being on the way, attaching to 辞書形 such as 「行く」, whereas 「最中に」 only says that an activity is in progress and does not attach to 辞書形. When a sudden event occurs during an activity, the two can be interchangeable, but 「最中に」 places more emphasis on being interrupted.
• ところをN2With 「ところを」, the second clause is an action directed right at that scene, such as being seen or being stopped, or the pattern is used for polite thanks and apologies, whereas with 「最中に」 the second clause is usually a chance interruption. 「ところを」 can also attach to 辞書形, た形, 「意志形＋とした」, and more.
• ているところN4「ているところ」 attaches only to verbs and cannot take 「名詞＋の」, and it is often used as 「ているところだ」 to describe one's own state at that moment. When taking 「に／を」, it can also indicate that something happens while you are in the middle of doing something, but it does not convey the nuance of unexpected interruption as strongly as 「最中に」.
• 〜たところだN3「最中に」 indicates that an action is in progress and not yet finished, attaching to 「名詞の」 or the 「ている」 form, with the second clause usually a sudden event; 「〜たところだ」 indicates that an action has just been completed and takes た形 as the predicate. The two mark different points in time and cannot be interchanged.

最中に marks the height of an ongoing activity that an unexpected event interrupts, whereas 間に marks something happening within a time span, 途中に／途中で marks a point partway through a process or route, and 〜たところだ means an action has just been completed. ところに／ところへ focus on someone or something arriving at a particular moment, ところを is followed by verbs acting on that moment (such as being seen or helped), and ているところ simply states that an action is in progress.