Abstract: In order to fill a container having a check valve with a spontaneously foaming filling material, the latter is introduced through the check valve already mounted at the container. In a piston arrangement as the filling device, filling material to be filled in is sucked into a mixing chamber (65) by means of a metering piston (60), foam gas is injected through the piston rod (58) of this piston, filling material and gas are mixed by means of a mixing piston (56), then forced by means of the metering piston (60) through the check valve into the container. This prevents frothing of the filling material, intermixed with foam gas, at any point in time between mixing and filling. During mixing, slight foaming is desirable in order to increase the absorption capacity for the foam gas.