In the process of photosynthesis, carbohydrase was externally secreted when the cells of Chlorella vulgaris were cultivated on Arnon medium. Especially when starch was added to the culture medium, induced enzyme was secreted during the course of cultivation. By its secreted carbohydrase, the multiplication of chlorella was promoted. The culture supernatant of chlorella was lyophilized and used as the source of raw carbohydrase. The carbohydrase activity was measured by the method of blue value and the properties of the enzyme were investigated. The optimal pH was 7.0 and the optimal temperature was 35°. Significant amount of enzyme activity was lost at more than 50° and the action of the substrate-protection was observed. As the effect of metal salts, the activity was increased in the presence of 3.3×10^-4M of Ca²⁺, Mg²⁺, Fe³⁺, Mn²⁺ and Fe²⁺, and inhibited in 3.3×10^-4M of Ag⁺ and Hg²⁺. Michaelis constant, Km, was 1.2 mg/ml to amylose as substrate. The effect of carbohydrase on chlorella cultivation was investigated. When cultivation of chlorella was performed in the presence of starch and purified amylase, its yield in dry weight was 3.5 times as much as that of standard. When coexistent bacilli with chlorella was counted on agar plates, its numbers ; bacilli : chlorella cells=1.79 : 10⁸ and those bacilli did not secreted carbohydrase.