For the investigation of the influence of microstructure of optical thin films on the in-plane thermal diffusivity of glass substrate coated by that films, we constructed the AC calorimeter which uses argon-ion laser as a thermal source and measured several kinds of ZnS optical thin films which prepared by the changing of thermal evaporation speed such as 5$AA$/s, 10$AA$/s, 20$AA$/s, 40$AA$/s, 60$AA$/s. The result showed that the thermal diffusivity decreases as the temperature increases. Also, when the evaporation speed is 20$AA$/s, the thermal diffusivity has maximum value, and the variation of the thermal diffusivities are 27% at maximum.