PURPOSE: To measure a temperature of a condensing lens in a stable thermal equilibrium state by placing a shielding plate in a focus of a laser beam between the condensing lens and an integrating sphere.

CONSTITUTION: A condensing lens 1, a scattering reflected light shielding plate 2 having a pin hole, and an integrating sphere 3 are placed in a laser optical path. By moving one of said three along the optical axis, the shielding plate 2 is placed in a focal position of a laser beam, only the laser beam is allowed to pass through, and also, shielding is executed so that the condensing lens is not irradiated by the scattering reflected light. Also, the integrating sphere is placed so as to receive through an aperture 11 of the laser beam which has passed through the pin hole of the shielding plate 2. Under this arrangement, the laser beam is led into the condensing lens 1 being an object to be measured, its passing light is received by the integrating sphere 3 through the shielding plate 2, and a temperature variation of the condensing lens 1 is detected by a thermocouple. Subsequently, this detected value and a detector signal of the integrating sphere are sent to a data logger, and the energy absorption factor of the condensing lens is derived.