In order to study on the conversion of provitamin D into vitamin D in vitro by sunlight, a procedure for irradiating an ergosterol (a provitamin D) solution and determining vitamin D_2 in the irradiated solution was proposed. Twenty-five ml of an ergosterol solution in ethanol (0.5 mg/ml) containing di-tert-butylhydroxytoluene as an antioxidant (0.1 mg/ml) was exactly taken in a quartz-made erlenmeyer flask and allowed to stand at an outdoor place between 11 and 14 o'clock for the sake of irradiating by sunlight. The yield of vitamin D_2 in the irradiated solution was determined according to the method applying the gas-liquid chromatographic procedure as reported previously. This method was confirmed to be an excellent one because of showing accuracy and good reproducibility even though yield was about 2%, and therefore it must be available for the author's purpose.