To provide a method for producing a simple reducer made of a fiber-reinforced resin, which does not need a mold for every caliber combination and can cope with a prescribed caliber combination by one mold.

In the method, the mold including a truncated cone-shaped mold body having a tapered surface of a funnel-shaped cross section squeezed from the maximum caliber to be used to the minimum caliber and rotary shaft extending from both ends of the mold body is prepared, a fiber reinforced base material impregnated with a resin by a filament winding method is wound onto the mold to surround the mold while the rotary shaft is rotated and demolded after being molded by solidifying or curing the resin, a funnel-shaped tubular body is produced, and next a straight pipe approximately matching the caliber is connected to both ends of the funnel-shaped tubular body.