PURPOSE: To expand the working range by selecting a work mode in which the first and second booms expanded/thrunk in multiple stages are linearly fixed for the work and a work mode in which the first boom is erected nearly upright and the second boom is laterally protruded for the work.

CONSTITUTION: The first boom 4 constituted of multi-stage booms expanded/ shrunk in multiple stages is erectably provided on an upper revolving superstructure 3 rotatably provided on a self-propelled vehicle body 1, the base end section of the second boom 6 constituted of multi-stage booms likewise expanded/shrunk in multiple stages is pivotally fitted to the tip section of the first boom 4, and the second boom 6 is erected or prostrated by a boom cylinder 7. The work mode A is selected by a work mode selecting switch for the normal crane work, and the first and second booms 4, 6 are extended into a nearly straight line for the work at a high position. When the work mode B is selected, the first boom 4 is erected nearly upright, and the second boom 6 is laterally protruded to expand the working range.