Heat conduction of semitransparent materials such as glasses consists not only of diffusive heat conduction but also radiative heat conduction. It was confirmed that the heat conduction equation approximating thermal conduction of a semitransparent material by 1-dimensional thermal conduction is a differential equation of 4th order in space and 1st order in time. The temperature equation for three layer material with semitransparent material sandwiched between opaque materials was derived using this heat conduction differential equation in Laplace space and the temperature equation is the sum of the temperature equations of the diffusive heat transfer region and the radiative heat transfer region. By using this equation, the rear surface temperature was obtained in the case that the front surface was pulse heated. Apparent thermal diffusivity for these data was determined using thermal diffusivity analysis method of flash method. The optical thickness region where the apparent thermal diffusivity deviation from the diffusive thermal diffusivity is 10% was obtained. The dependence of the apparent thermal diffusivity on the both sides emissivity of semitransparent material was confirmed.