To provide a detergent for a molding die superior in removing capacity of a deposit adhered to a molding die of a styrenic resin and its cleaning method.

A descaling detergent with pH of 1-5 is put in an impregnation pool 11, and the molding die of the styrenic resin is put in and left there, thereby the deposit adhered to the molding die is removed and then the molding die is taken out from the impregnation pool and washed with water. Successively, a styrenic resin detergent comprising a styrene resolvent and a styrene unresolvent is put in another impregnation pool 11, and the molding die for the styrenic resin in the impregnation pool 11 and left as it is, so that the deposit adhering to the molding die is released from the molding die and settled in the descaling detergent. After that, the molding die is taken out from the impregnation pool 11 and is washed with water, thus the deposit adhering to the molding die is removed.