A method for assaying cellulase activity, including: a process (A) of preparing two or more substrate solutions, which have an identical absorbance measured at an identical wavelength and in which cellulose is dispersed at an identical concentration, and measuring the absorbance of each of the substrate solutions; a process (B) of respectively adding different kinds of enzyme solutions to the substrate solutions, and performing an enzyme reaction under the same conditions; a process (C) of measuring the absorbance of each of the substrate solutions after the enzyme reaction; a process (D) of calculating the absorbance decrease values of the substrate solutions before and after the enzyme reaction; and a process (E) of assaying cellulase activities of the enzyme solutions based on the absorbance decrease values. In the assay of the process (E), it is determined that the enzyme solutions have higher cellulase activity as the absorbance decrease values become larger.