The method of the present invention for producing β-chitin comprises a step of cultivating an alga capable of producing β-chitin using a medium that contains nitrogen (N), phosphorus (P), silicon (Si), and selenium (Se), wherein the weight ratio of phosphorus to nitrogen (P/N) is 0.1 or above, the weight ratio of silicon to nitrogen (Si/N) is 0.1 or above, and nitrogen (N) is contained at 160 ppm or more. It is preferable that selenium (Se) is present in the medium at a concentration of 0.08 ppm to 0.008 ppb.