A measuring instrument for thermal diffusivity of thin films with thickness of around 1 µm has been developed based on a pulsed light heating thermoreflectance technique. The instrument observes transient temperature change at the surface of the film with 1 ns time resolution. One face of the substrate side of the thin film is heated by pulsed laser with the duration of 2 ns, and the temperature rise at the opposite face is detected by sensing reflectivity change of the film. For the observation of the reflectivity change of the film, a continuous wave laser diode is used instead of a pulsed laser for the conventional thermoreflectance measurements. We measured the thermal diffusivity of a titanium nitride film (680 nm) using the developed instrument, which coincided with the data obtained by the standard instrument of NMIJ, AIST. The developed instrument has several features: desktop size and short data acquisition time about 1 minute.