To measure formaldehyde selectively by a device excellent in portability.

A specimen collecting part 8, a specimen disassembling part 10, and a gas detecting part 12 are arranged along a flow path from an inlet to suck atmospheric air, and a nitrogen gas cylinder is connected to a line on the upstream side of the specimen collecting part 8 through a three-way solenoid valve 14. For the specimen collecting part 8, a collector in which polymer adsorbing agent TENAX is filled up is used. Also the specimen disassembling part 10 comprises a cell 10a into which formaldehyde separated from the specimen collecting part 8 is introduced and a mercury lamp 10b which irradiates ultraviolet ray in the cell 10a. In addition, for the gas detecting part 12, an FET type gas sensor (Pd-MISFET) as a palladium film type electrode detecting mechanism is used.