To provide a method for producing a hollow molding which makes the strength of the joint of half-hollow moldings enough high, prevents a molding process from being complex during secondary molding, and does not need a press material for pressing the joint in particular.

With the use of a slide mold (30), a fixed mold (1), and first and second slide cores (11 and 12) attached to the fixed mold, first and second half-moldings (A1 and A2) and a third half-molding (B) are molded integrally to have joining end surfaces (ab and aa). With the third half-molding (B) made to remain, the slide mold (30) is slid toward the first and second slide cores (11 and 12) to adjust the respective joining end surfaces, heating surfaces (41-43) are inserted between them and retracted after melting the joining end surfaces, and the molds are closed to weld the joining end surfaces (ab and aa) together.