This paper describes an analysis for the membrane dynamics and deformation of solar power sail demonstrator "IKAROS". After the successful deployment of membrane, some images of the whole sail membrane of IKAROS were taken by separation cameras. From these data, it has estimated that the membrane of IKAROS had a deformation different from the prediction considering the solar radiation pressure. In this paper, these observed deformations of membrane are reported and compared with the results of numerical simulation using multi-particle model. A membrane's deformation over time observed in the images taken by side monitor cameras is also reported and the cause of the changing is discussed.