PURPOSE: To efficiently obtain the titled sheet having significantly fine and dense pores and good surface gloss also substantially improved in surface smoothness, by incorporating a polyolefin resin with a specific coated foaming agent with a particle size below a specified value.

CONSTITUTION: A combination of inorganic carbon dioxide generator and weak acid is used as the foaming agent. Either or both of these components is coated with 5W50wt.% of a coating with a melting point of 40W120°C followed by pulverization to a size of ≤60 mesh. The resulting particles are incorporated in a polyolefin resin followed by extrusion foaming at temperatures higher than the softening point of the resin to obtain the objective sheet. Said foaming agent is to be a combination of sodium bicarbonate and citric acid, while the coating a wax, fatty acid ester of polyhydric alcohol or fatty acid amide.