PURPOSE: To improve maintainability by simplifying work for changing a filter while improving assembling performance.

CONSTITUTION: In a constitution, a filter 29, which is positioned behind an operator's seat 12 to provide an outside air introducing port 28 drilled in a floor plate 22 and positioned in a lower side of the floor plate 22 to cover the outside air introducing port 28 from the outside, is removably provided. In this way, an inside/outside air switching part 26 for connecting the outside air introducing port 28 to an air conditioner main unit is physically separated from an operator's cabin main body 23, and an air conditioner 24 can be set up before mounting the operator's cabin main body 23 onto the floor plate 22. In the case of clogging of the filter 29, a hand is inserted into the lower side of the floor plate 22 by a worker, and the filter 29 is taken out from inside a filter box 31 and replaced with a new one.