A piston supporting structure for a linear compressor includes a piston reciprocating in the axial direction in response to the driving force of a motor, a first spring of which one end portion is fixed to one side of the piston, and a second spring of which one end portion is fixed to the other side of the piston. During a first period of time, a first electrode force is applied to the workpieces to be welded, and a weld current is applied that causes heating of an associated workpiece-to-workpiece faying surface, the first electrode force being applied at a first electrode stroke. A laser beam of which intensity is modulated in accordance with information to be recorded is irradiated onto this recording layer to cause a state change in the recording assist material andor the dielectric material. The look-up engine transfers the second selector to the CAM and receives a corresponding second result from the CAM.