A method for forming a belt-like rubber sheet S by extruding rubber on a drum outer peripheral surface 3a by an extruder 2 includes: a front end forming step for forming a front end S1 having a wedge-shaped cross section; an intermediate part forming step for forming an intermediate part S2 having a desired thickness; and a rear end forming step for forming a rear end S3 having a wedge-shaped cross section by gradually decreasing an extrusion amount of the rubber to zero from a predetermined extrusion amount in a predetermined time, gradually decreasing the rotational speed of the drum outer peripheral surface 3a to zero from a predetermined rotational speed in the predetermined time, and gradually decreasing a distance from the drum outer peripheral surface 3a to a die 21 to an extrusion end distance from a predetermined distance in the predetermined time.