To provide a plastic judging device unlikely to be influenced by the temperature and humidity and capable of sensing a pyrolysis gas stably.

The judging device is to judge the plastic contained in a test object by sensing the sort of the pyrolysis gas generated by pyrolysis of the plastic using a gas sensor, whereby inside the device is arranged capable of being air-cooled by introducing the atmosphere or the pyrolysis gas is introduced to the sensing surface of the gas sensor and the atmosphere is introduced to that portion of gas sensor excluding the sensing surface. The pyrolysis gas introduced to the sensing surface is subjected to sensing of its gas sort, and the temperature of the flow passage from the pyrolyzing part to the suction port and the temperature of the sensor are raised so that bedewing is prevented. The introduced atmosphere cools the temperature raised by the pyrolysis gas and suppresses an excessive temperature rise of the sensor and/or its holder so as to prevent a drop of the sensor performance and a deterioration of the sensor. This allows stable sensing of the pyrolysis gas where the temperature and humidity are unlikely to give influence.