To easily and safely obtain a transparent liquid soap by reacting a waste edible oil with a saponifier in an electric heating pot.

This method for producing a soap from a waste edible oil comprises reacting (A) the waste edible oil, e.g. rapeseed oil or soybean oil, with (B) a saponifier. The method prevents an operator from smelling a malodor generated on the saponification, releases the operator from heavy labor of stirring, enables a housewife to easily produce a transparent liquid soap in an ordinary home only by leaving the resultant liquid at rest, and is useful for recycling resources.