PURPOSE: To inject a liquefied gas for foaming into a foaming resin without interruption.

CONSTITUTION: With possibility of switching, a plurality of gas containers 1 are coupled with a gas supplying path 3, with which an intermediate tank 2 is coupled. With this intermediate tank 2, a gas injecting path 5 is coupled for injecting liquefied gas G in the tank into a foaming resin. The stored quantity in the tank 2 is sensed by a sensing means 6, and if the quantity sinks below a specified level, the gas container 1 in supplying the liquefied gas G to the intermediate tank 2 is changed over by a changeover means 4 which is interposed in the gas supplying path 3.