To allow a cargo space as a loading space for long materials by expanding a part of a cabin rear plate to a cabin side and integrally forming an arm rest in the cabin.

An arm rest 20 is formed expandingly toward a cabin side, in a right and left central lower part of a flat rear wall portion 14 of a cabin rear plate 12. This arm rest 20 has a hollow square cylindrical shape whose tip is closed and projects into the cabin, and a cushion material 21 is laminated on the flat upper surface to form an arm rest surface. The hollow part in this arm rest is opened on a carriage 2, and a longitudinally long loading space 22, which connects with a cargo receiving surface and is recessed on the cabin side, can be formed on the carriage 2. This loading space 22 can receive part of long materials L loaded on the cargo receiving surface to allow load of the long materials L.