PURPOSE: To obtain a resin foaming unit of uniform quality with sufficient strength by spraying a predetermined amount of material beads in the bottom part of the hollow ground and liftably arranging a mold form on the material beads thereafter adjusting a flow amount of foaming steam and its pressure to an amount suited for foaming in the initial, intermediate and final periods.

CONSTITUTION: A steam blow pipe 3 of about 40mm diameter is buried in an about 60mm sand layer 4 on the hollow ground 2 of about 1m×1m×80cm volume. After resin beads 10 are sprayed in about 23mm thickness on the sand layer 4, an about 30mm space part 11 is set between the sand layer 4 and a liftable mold form 12 by inserting it to struts 5, 6 whose one side serves as a pipe. Next, a flow amount is adjusted to 0.1m3 or more per minute in the foaming initial period, 0.1m3 or less per minute in the intermediate period and again 0.1m3 or more per minute in the final period while lifting the mold form 12 at a speed of about 5cm per minute by supplying steam of temperature about 100°C to the steam blow-in pipe 3. In this way, a foaming resin unit of uniform quality as a whole can be obtained.