To manufacture a belt main body uniform in thickness and to form a thick-walled slippage preventing members to both ends of the belt main body at the same time to enhance the strength of the belt main body.

The leveling roller 43, which is formed so as to be made different in level at both end parts thereof and has not only a roller 43a having a large diameter provided to the central part thereof but also rollers 43b having a small diameter at both end parts thereof, of a core mold 42 is located by the rotary bodies 44 guided by the guide parts 41a provided to both internal ends of a coating mold 41 to be held in a freely rotatable manner. The coating solution applied to the inner peripheral surface of the coating mold 41 is leveled by the leveling roller 43 of the core mold 42 while rotating the coating mold 41 to form a coating film 34 having thick-walled parts having a constant width in the peripheral direction of both ends thereof from the central part thereof.