PROBLEM TO BE SOLVED : To provide a technique which can proliferate a stem cell while maintaining the undifferentiated state.SOLUTION : The present invention provides an agent for maintaining the undifferentiated state of a stem cell or an agent for promoting its proliferation, comprising a Ganoderma lucidum subcritical water processed product or an extract thereof as an active ingredient. Specifically, the present invention can be used as a method for preparing a stem cell, and/or an agent for maintaining the undifferentiated state of a stem cell or an agent for promoting its proliferation, in regenerative medicines and regenerative cosmetics.SELECTED DRAWING : None