To enable a trace amount of moisture in a dried air and a low-dew-point gas to be dehumidified, to enable the presence of the infinitesimal amount of moisture to be sensed easily by changing its color sensitively for a short time by utilizing moisture adsorption, to enable its reproduction to be performed without heating at high temperature, and to enable method therefor to be carried out at low cost.

A silica gel is used as an adsorbing substrate, and magnesium perchlorate and an acid-base indicator are deposited on the silica gel as a moisture absorption accelerator and a moisture indicator, respectively. Bromocresol purple or bromothymol blue is used as the acid-base indicator. The granularity of the silica gel is 180 μm or smaller, and respective deposited amounts of the magnesium perchlorate and the moisture indicator are 1-20 parts by mass and 0.001-1.0 parts by mass with respect to the silica gel of 100 parts by mass.

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