This paper deals with a transmission loss of sandwich plates which consist of two flat glass plates and a laminate film. It takes a very huge time to measure a transmission loss in a reverberate room. Therefore, the development of evaluating for a transmission loss by numerical calculation is very useful for a soundproofing design. In this paper, at first, a loss factor for the complex shear modulus of three kinds of laminate films are measured by the authors' experiment, and then, sound impedances of the films are calculated. A transmission loss of a random incident is calculated with the next conditions : pressure and velosity boundary conditions between air and glass, pressure and velosity boundary conditions between glass and film, and by using equations between a pressure and a sound impedance. The calculation results show good agreement with the experimental results measured in a reverberate room.