Photomechanical materials in which the shape change of a high polymer is extracted as motion by photoirradiation are attracting attention lately. Nevertheless, details of the complicated mechanism of this photoirradiation-induced shape change have yet to be elucidated. Using molecular dynamics and the Monte Carlo method, we have conducted molecular simulations for a case in which a straight chain oligomer is irradiated by photons. Thereby, we have investigated the interaction between an oligomer and photon as the first step in elucidating photoirradiation-induced shape change. Results show that the oligomer's light absorption and the Compton effect between a photon and an oligomer cause a chemical energy change and bring about shape change. Furthermore, short-wavelength ultraviolet radiation has been found to cause greater shape change than infrared radiation does.