To precisely measure the pollution rate of a test window for shielding a fire detection sensor.

An outgoing light H43 of a light source 43 is incident on a detection sensor 45 through test windows 21-1 and 23-1, and the light receiving quantity by the detection sensor 45 is compared with an initial value to calculate the total transmittance Y of the test windows 21-1 and 23-1 (=ab, a; transmittance of the test window 21-1=1-pollution rate, b; transmittance of the test window 23-1). An outgoing light H46 of a light source 46 is similarly incident on a detection sensor 47 through test windows 23-1 and 23-2 to calculate the total transmittance Z of the test windows 23-1 and 23-2 (=bc, c; transmittance of the test window 23-2), and an outgoing light H48 of a light source 48 is further incident on a detection sensor 48 through the test windows 23-2 and 21-1 to calculate the total transmittance X of the test windows 23-2 and 21-1 (=ca). The pollution rate (1-a) of the test window 21-1 is calculated based on the equation [1-a=1-(XY/Z)1/2].