Carry out a vapor etching step of cleaning an inside of a chamber of a vapor phase growth apparatus by vapor etching using HCl gas (S1). Carry out an annealing step of sequentially annealing a predetermined number of silicon wafers, one by one, in a non-oxidizing atmosphere (S2, S3). Repeat the vapor etching step and the annealing step a prescribed number of times. After having carried out the vapor etching step and the annealing step the prescribed number of times (S4: Yes), collect contaminants on the surface of each of the wafers, and measure the Mo concentration using ICP-MS (S5). Evaluate the cleanliness of the vapor phase growth apparatus on the basis of each Mo concentration value and the relationship between the Mo concentrations (S6). Thus, provided is a method with which it is possible to measure, with high sensitivity, the contamination amount of a vapor phase growth apparatus.