PURPOSE: To form highly accurately and efficiently an insert belt stripe without deforming the same, by a method wherein a movable roller is made to move, according to movement of a movable mold, on a predetermined route in a reciprocating state by abutting against the insert belt stripe while constant tension force is being applied to the same.

CONSTITUTION: When a movable mold 34 is moved after completion of molding, a cylinder is driven, and a first movable roller 44 and a second movable roller 46 are moved in the direction separating from the movable mold 34 while it is being abutted against an insert belt stripe 40. As the insert belt stripe 40 is strained this time when the movable mold 34 is moved in a clamping direction for the next cycle, a first and second movable rollers 44, 46 are moved in the direction of the movable mold 34 while they are being abutted against the insert belt strip 40 so that they absorb this tension amount. With this construction, as the molding can be performed without necessitating to provide any slack on the insert belt stripe and moving the insert belt stripe at the time of breaking and clamping of the molding die, positioning accuracy of the insert belt stripe is improved and deformation of the insert belt stripe can be prevented.