To produce a soap with a very high viscosity like a rice cake.

A solid raw material soap comprises 5% or more of glycerine, 10% or more of propylene glycol, 10% or more of sodium stearate, 5% or more of sodium laurate, 5% or more of sodium polyoxyethylene lauryl ether sulfate, 0.5% or more of lauryl sulfate, and 25% or more of water, by weight. A solid salt comprises 2% or more of sodium chloride, 20% or more of potassium chloride, and 20% or more of magnesium chloride, by weight. The solid salt is dissolved in water to prepare a saturated solution of the salt. The raw material soap is heated to a temperature equal to the melting point thereof or higher to melt it. The saturated solution of the salt is mixed with the melted raw material soap, in a ratio by weight of the saturated solution to the raw material soap of 3-11 to 100, followed by cooling the mixture to room temperature.