To provide an immersion treatment method with a hygroscopic chemical which can control the rising of the moisture content of the hygroscopic chemical without replacement with a fresh treating solution.

The immersion treatment method employs a hygroscopic chemical, e.g. a coating containing methanol or a cleaning solution and comprises carrying in a target article to an immersion treatment station provided with an immersion vessel 12, subjecting the article to an immersion treatment with a hygroscopic chemical stored in the immersion vessel and carrying out the article from the immersion treatment chamber. Wet air in the air layer A on the hygroscopic chemical is replaced with dry air conditioned by dehumidifying the atmosphere so as to keep the air layer A on the hygroscopic chemical at a set absolute humidity.