PURPOSE: To obtain a device for predicting a life of an adsorbent which eliminates an acid gas which is generated within a clean room.

CONSTITUTION: A dust-eliminating filter 16 is provided on the downstream side blow-off port of a fan 14 which is installed within a casing 12 at a fan filter unit 10 and a filling container 24 of a color-developing agent which reacts with an acid gas is installed on the upstream or a downstream side of an adsorbent 20 in the fan filter unit 10 where the adsorbent 20 is installed at the intake port. A gas transmission film 32 such as teflon is provided at the filling container 24, thus constituting a life-prediction device of the adsorbent 20. The filling container 24 with the gas transmission film 32 of the acid gas and an observation window 36 is installed on the upstream or downstream side of the adsorbent 20, a color-development change of a color-development agent 34 which is filled into the container 24 is measured by a detection means such as a light-adsorption photometric meter, an amount of adsorption of acid gas to the adsorbent 20 is calculated from the output signal, and then a remaining amount of adsorption is calculated from a ratio in reference to the amount of saturated adsorption of the adsorbent 20.