Disclosed is a method for measuring the thermal diffusivity in the thickness direction of a thin sample plate, which has the steps of forming a conductive thin layer on at least one surface of said thin sample plate to allow the thin film to function as an ac heater generating joule heat, applying an ac current modulated with a given modulation frequency to the ac heater (conductive thin layer) so as to produce ac joule-heating, generating an oscillation response corresponding to said ac joule-heating on the opposite surface of the ac heater, and measuring the phase shift between said oscillation response, and the ac joule-heating, thereby obtaining the thermal diffusivity in the thickness direction of said sample plate. Apparatus useful for the method are also disclosed.