To extend a visual field along the front from the side of a boom side of a driver's seat by setting a boom upper surface position on the extreme outside at the time of prostrating a multistage telescopic boom on the side of a driver's cab and below the level of an eye of a driver and rounding off a corner of an upper part on the opposite side of the driver's seat of a boom on the extreme outside.

A boom 40 is installed on a revolving frame at a part of a boom foot 41 by a boom foot pin free to rotate. As this installing position is in the rear of a revolving center of an upper part revolving body and a boom support part of a lower part traveling body is arranged low, a head end of the boom becomes low at the time of making the boom 40 in a prostrating state, and a visual field along the front from the left side of the driver's seat is extended. Additionally, as a cross-section shape of a boom 40a on the extreme outside is shaped with the corner on the upper part rounded off, the visual field from the driver's seat is further extended.