Agent for promoting undifferentiation and cryoprotective agent using aprotic zwitterion

A medium for cryopreservation, comprising an aqueous solution of an aprotic zwitterion represented by formula (4):

• R4-X—R2-A (4)
• wherein R4 is an alkyl group having 1 to 7 carbon atoms, an alkenyl group having 1 to 7 carbon atoms, or an alkyl group having 1 to 7 carbon atoms and containing an ether bond, X is a cation moiety of the zwitterion and represents a cation selected from the group consisting of an imidazolium cation, a phosphonium cation, a sulfonium cation, a pyrazolium cation, a pyridinium cation, a pyrrolidinium cation, a morpholinium cation, a cyclopropenylium cation and a piperidinium cation, A represents an anion moiety and represents an anion selected from the group consisting of SO3−, —COO−, —OP═O(H)O−, —OP═O(CH3)O− and —OP═O(OR3)O−, R2 is an alkylene group having 1 to 5 carbon atoms and optionally having a substituent, and R3 is hydrogen or an alkyl group optionally having a heteroatom in the molecular chain, and a cell-permeable substance, wherein the amount of the cell-permeable substance is from 1 to 30 parts by weight per 100 parts by weight of the aqueous solution of the aprotic zwitterion.