PURPOSE: To produce a thermoplastic resin foam having a high expansion ratio, containing uniform and fine cells, being free from discoloration and malodor, and having a smooth surface by using a clean blowing agent such as an inorganic gas which is inexpensive and does not cause any environmental problem.

CONSTITUTION: The foam is obtained by melt-kneading a resin composition comprising a thermoplastic resin and a fine powder having water of crystallization and a mean particle diameter of 0.1-500μm, injecting an inorganic gas into the melt, and releasing the pressure from the system. Examples of the foaming processes adopted include extrusion foaming with an extruder and batch foaming in a pressure container.