To provide a technology of finding out deviation of a blade, caused by an internal factor such as deformation or the like of the blade, under an operating state.

A rotary blade diagnostic device (10) includes: image pickup means (11) picking up an image of a revolving travel locus (27) of a blade (25) at a high speed during rotation of the blade (25) of a wind power generator (20); a travel locus database (14) with the revolving travel locus (27) of the blade (25) being picked up by the image pickup means (11) and preliminarily accumulated; data on the revolving travel locus (27) accumulated in the travel locus database (14); verifying means (36) verifying the revolving travel locus (27) picked up by the image pickup means (11); and verified result output means (37) outputting a verified result from the verifying means (36).