Abstract: A method of manufacturing a molded plastic product prepares a core and a skin. The core has a pad supporting face, a sidewall surrounding the supporting face, and a pad housing defined by the supporting face and sidewall. The skin has a side face to be fitted to the sidewall of the core. The method sets the core on an upper mold so that the pad supporting face is oriented toward a lower mold, and places the skin on the lower mold so that the back face of the skin is oriented toward the pad supporting face. The method closes the upper and lower molds and injects foam ingredients between the core and the skin. The foam ingredients is reacted and adheres to the skin to form the pad as well as to the pad housing, to thereby integrate them into one body. The method is characterized by arranging an elastic seal on the side face of the skin so that the side face of the skin is attached to the sidewall of the pad housing through the elastic seal by the pressure of the reacted foam ingredients.