To prevent skids from being lifted up together with a work by firmly integrating each of the skids onto a first and a second mounting plates.

A plurality of first recessed parts 35 are provided at an upper edge of a first mounting plate 27, and a plurality of second recessed parts 37 are provided at an upper edge of a second mounting plate 25; at the lower end of each of the skids 31, a first slit 39 to be inserted into a corresponding lower part of the first recessed part 35 in the first mounting plate 27 and a second slit 41 to be inserted into a corresponding lower part of the second recessed part 37 in the second mounting plate 29 are provided; a first engaging hole 43 is provided near each of the first recessed part 35, a first engaging pawl 43 engaging with the first engaging hole 43 is provided in each of the skids 31, a second engaging hole 47 is provided near each of the second recessed part 37, and a second engaging pawl 49 engaging with the second engaging hole 47 in each of the skids 31 is provided.