For producing a liquefied material from a glycerin-based solid material, a glycerin-based solid material is allowed to be efficiently liquefied to produce a liquefied material that can be used as, for example, a fuel or the like. A glycerin-based solid material, such as a glycerin-based solid material produced as a byproduct in the production of a biodiesel fuel from a plant oil, is mixed with at least one oil selected from the group consisting of a mineral oil, an animal oil, and a plant oil; and an acid to produce a liquefied material, the acid being mixed in such that the pH of the liquefied material is 3 to 12.