1. A method for evaluating the toxicity of a chemical by using an alga, comprising: (a) a thawing step of thawing frozen algal cells by heating, and diluting the obtained suspension of the cells by adding a culture medium thereto; (b) a recovery culture step of culturing the algal cells obtained in the thawing step (a) to allow the algal cells to recover from the effects of freezing and thawing; (c) a confirmation step of collecting a part of the algal cells after the recovery culture step (b), diluting the part of the algal cells by adding a culture medium thereto, and measuring the amount of the luminescence of the delayed luminescence of the algal cells as initial value data; (d) an exposure step of mixing the algal cells after the confirmation step (c) with a solution containing a test substance to prepare an exposure sample, and culturing the exposure sample; (e) a measurement step of measuring the amount of the luminescence of the delayed luminescence of the exposure sample after the exposure step (d) as exposure data; and (f) an evaluation step of calculating an evaluation value based on the initial value data and the exposure data, and evaluating the toxicity of the test substance based on the evaluation value.