PURPOSE: To limit an area for producing mushrooms, simultaneously prevent a culture medium from drying and obtain large-sized high-quality mushrooms in high yield by shielding a culture medium in a state of close contact with a container for cultivation from light during a cultivation period and irradiating only parts required to produce the mushrooms with light.

CONSTITUTION: A culture medium is filled in a cultivation bag 2 (e.g. made of polypropylene) having a light shielding bag 1 (e.g. made of black polyethylene) mounted thereon by folding the top of an inner bag 1 so as to be in close contact with the top surface of the culture medium by a width (T) 5 to 15mm). The mouth in the center of the top is squeezed thereinto to close the bag with a plug 3. Inocula are than inoculated into the aforementioned culture medium and cultured. Thereby, production of deformed mushrooms readily occurring in the gap between the container and the baglike material can be eliminated.