Even in the case of a sample exhibiting low photoresponse, such as a wide bandgap semiconductor, a measurement method which enables highly accurate CPM measurement is provided. When CPM measurement is performed, photoexcited carriers which are generated by light irradiation of a sample exhibiting low photoresponse such as a wide bandgap semiconductor are instantly removed by application of positive bias voltage to a third electrode which is provided in the sample in addition to two electrodes used for measurement. When the photoexcited carriers are removed, even in the case of the sample exhibiting low photoresponse, the controllability of a photocurrent value is improved and CPM measurement can be performed accurately.