PURPOSE: To measure the quantity of atomic oxygen irradiation of a polyimide film made in the space by irradiating the film with atomic oxygen both on the ground and in the space, and using the test data of determining how the quantity of irradiation is related to the amount of mass decrease per unit area.

CONSTITUTION: A polyimide film 11 as a sample is affixed to the surface of a crystal for sensing 12. A current is fed to the resistance of a heater 13 so that the temps. of the crystal 12 and a reference crystal 14 are held constant. An oscillator for sensing 15 oscillates the crystal 12 while an oscillator 16 for reference oscillates the reference crystal 14. A mass change calculating part 17 calculates the change amount of the mass of the sample from the difference in the frequency between the crystals 12 and 14. A communication device 18 transmits the obtained change of mass to the ground. On the ground it is possible to determine the mass decrease in the sample from this change amount of mass, and the change with time in the quantity of irradiation with atomic oxygen can be known from the change with time of the mass.