To facilitate breakage prevention and thrust adjustment of a skirt guard safety device.

When thrust measured by a spring scale is higher than estimated thrust, an adjuster 16 with attractive force is pasted in front of the operator 14a of a safety switch 14, or on the back of a skirt guard 6, to narrow a gap G between the skirt guard 6 and the operator 14a of the safety switch 14 for reducing thrust. The adjuster 16 is a seal with attractive force, having a thickness of 1 mm or so.