To facilitate combining work for retractably holding a boom in a boom rest.

A boom operation control device 40 comprises a boom position calculating circuit 51 for calculating position of a telescoping boom based upon detected values from a revolving angle sensor 41 and a derricking angle sensor 43, a boom position determining circuit 53 for determining whether or not the calculated boom position is within a retreating region set in a memory 55, and a speed regulation circuit 57 for decreasing a speed of revolution or derricking of the telescoping boom when a length of the telescoping boom is detected to be in a wholly retreated state by a length sensor 45 and the telescoping boom is determined to be within the retreating region by the boom position determining circuit 53.