PURPOSE: To give the durability to a trim member and enable to manufacture even a fragile sheet without breaking by a method wherein reinforcing material is arranged at the peripheral edge part of a sheet in a cavity formed by a top force and a bottom force and the forces are clamped and resin is injected in the cavity.

CONSTITUTION: In the forming of a trim member on the peripheral edge part 11 of a sheet by injecting resin into a cavity 9 formed at the mating part of a top force 7 and a bottom force 8 under the state that the peripheral edge part 11 of the sheet is fronted through a sealing member into the cavity 9, the resin is injected after the arrangement of the reinforcing member on the peripheral edge part 11 of the sheet and the clamping of the force. In case that the sheet is a fragile sheet such as a glass sheet or the like, the clamping of the top force and the bottom force is preferably done under the pressure of 10kg/cm2 or less per the projected area of the fragile sheet. Thus, the breakage of the sheet at injection molding can nearly perfectly be prevented from developing.