In a deformation simulation of a member joined by spot welding, an effective width being a width in a direction intersecting a direction of a load centered on a spot welded portion (14) on a flange face (13a) where the spot welded portion (14) is provided of a member (10) and changing correspondingly to a change in the load is calculated every predetermined time interval, and fracture of the spot welded portion (14) is predicted using the calculated effective width. This configuration enables precise fracture prediction of a spot welded portion where spot welding is modeled, for example, in a case of performing collision deformation prediction of an automobile member on a computer.