In a semiconductor heat-dissipating substrate made of a Cu--W alloy whose pores have been infiltrated with copper, being a porous tungsten body whose pore diameter at a specific cumulative surface area of 95% is 0.3 .mu.m or more, and whose pore diameter at a specific cumulative surface area of 5% is 30 .mu.m or less, thermal conductivity of 210 W/mK or more is obtained by decreasing the content of iron-family metal to be less than 0.02 weight %. Likewise, changing the amount of infiltrated copper in a molded object by utilizing a multi-shaft press to vary the amount of vesicles in the middle and peripheral portions makes for offering at low cost a semiconductor heat-dissipating substrate that in between middle and peripheral portions made of different materials does not have bonding matter.