Process for producing blow-molded foam

A process for producing a blow-molded foam is described. The blow-molded foam includes foamed cells, an average cell diameter of the foamed cells in a thickness direction of a wall defining the blow-molded foam being less than 300 µm, a standard deviation of a cell diameter of the foamed cell in the thickness direction of the wall being less than 40 µm. The process comprise kneading a polyolefin-based resin containing a foaming agent mixed therewith in an extruder (35), and then retaining the polyolefin-based resin at a cylinder-shaped space (9) defined between a mandrel (7) and a die outer cylinder (8), and extruding a parison (12) between split mold blocks (13) from a die slit (11) at an extrusion rate of 700 kg/h or more by use of a ring-shaped piston (10) after a predetermined amount of the resin is retained at the cylinder-shaped space (9) and carrying out blow molding after clamping by blowing air into the parison.