Method of cultivating algae and photobioreactor

A method of cultivating algal cells of an algae belonging to a class selected from Chlorophyceae, Euglenophyceae, Bacillariophyceae and Haptophyceae, the method comprising:

• irradiating the algal cells with an artificial light having a ratio of (i) photon flux density in a wavelength range of 520-630 nm to (ii) photosynthetic photon flux density, that is 65% or more;
• and
• measuring a cell density of an algal cell culture provided by cultivating the algal cells,
• wherein the photon flux density in the wavelength range of 520-630 nm of the light irradiated to the algal cells is set to be a first photon flux density value when the cell density is a predetermined value or less, and is set to be a second photon flux density value, which is larger than the first photon flux density value, when the cell density is the predetermined value or more.