Foam board of polyolefin resin and method for its production

To provide a foam board of a polyolefin resin which has an excellent heat insulating property and sound absorption property, of which a product can be material-recycled and which can continuously be produced at a low cost; a method for producing a foam board, which comprises extrusion-foaming by ejecting a mixture of a polyolefin resin composition containing a linear polypropylene resin having a melt tension of from 5 to 30 g at 230° C. and a foaming agent containing at least carbon dioxide in the supercritical state, by means of an extruder which has a multi-hole type die having an opening diameter of from 0.1 to 1.0 mm and the number of openings of from 50 to 5,000 at an ejection amount (V) of from 0.05 to 0.45 kg/hr per one opening in the die at a melt-extrusion temperature of from 160 to 250° C., under atmosphere at a proximal resin pressure of from 7 to 20 MPa at the openings of the die.