PURPOSE: To readily prevent water evaporation of medium and to efficiently culture a protoplast without requiring an expensive device by arranging a water retaining material around a well of a plate for cell culture and adding water to the water retaining material.

CONSTITUTION: A water retaining material 5 is arranged around a well of a plate for cell culture and water is added to the water retaining material to culture cells in the well. For example, the water retaining material 5 comprises paper or agar gel, the amount thereof used is an amount to give about 1/3-twice as long as the depth of a medium 6 to be fed to the well and the total amount of the water retaining material is preferably larger than the amount of the medium.