The objective of the present invention is to provide a method for cultivating an alga having ability to produce alginic acid industrially and effectively and a method for producing an alginic acid-containing composition industrially advantageously. The method for cultivating an alga according to the present invention is characterized in comprising the step of cultivating the alga heterotrophically, wherein the alga belongs to the genus Parachlorella and has ability to produce alginic acid.
