[Object] To provide an excellent foaming agent which does not have problems in handling and operation (example: risk of explosion or fire) and inhibition of cross-linking resulting from a foaming agent and problems, such as, mold pollution and environmental pollution, caused by a foaming agent residue, which has excellent uniform dispersibility in a subject of foaming, and which can be used as an alternative to the chemical decomposition type foaming agent.[Solution] A foaming agent formed from at least (A) a high molecular weight compound having a saturated water absorption of 10 to 1,000 g/g in ion-exchanged water (25° C.) and (B) water, wherein a storage modulus (G′) of the agent, determined on the basis of a viscoelasticity measurement at a temperature of 20° C., is 8.0×101 to 1.0×106 Pa at a frequency of 5 rad/s.