A construction machine, comprising: a lower travel body; an upper swivel body mounted swivelably on the lower travel body; a swivel motor which hydraulically drives the upper swivel body to swivel the upper swivel body; a working attachment installed on the upper swivel body and including a boom capable of being raised and lowered, a boom cylinder which hydraulically raises and lowers the boom, an arm pivotably connected to a front end of the boom, and an arm cylinder which hydraulically causes the arm to pivot; a hydraulic actuator circuit which has a first circuit including the boom cylinder and a boom control valve for controlling an operation of the boom, a second circuit including the arm cylinder and an arm control valve for controlling an operation of the arm cylinder, and a third circuit including the swivel motor and a swivel control valve for controlling an operation of the swivel motor; a first pump which is a hydraulic pressure source of the first circuit; a second pump which is a hydraulic pressure source of the second circuit; a third pump which is a hydraulic pressure source of the third circuit; a merge valve which has a first position and a second position and which forms a fluid path for causing third pump fluid which is hydraulic fluid discharged by the third pump to merge into the first circuit in parallel with the swivel motor at the first position and prohibits the third pump fluid from the merge into the first circuit at the second position; and a merge selecting control section which controls switching of the position of the merge valve, wherein the merge selecting control section holds the merge valve at the first position, when a boom raising operation, which is an operation for moving the boom in a raising direction, is performed during the swivel operation, characterized in that the merge selecting control section holds the merge valve at the first position also when only a swivel operation, which is an operation for the swivel motor, is performed. a lower travel body; an upper swivel body mounted swivelably on the lower travel body; a swivel motor which hydraulically drives the upper swivel body to swivel the upper swivel body; a working attachment installed on the upper swivel body and including a boom capable of being raised and lowered, a boom cylinder which hydraulically raises and lowers the boom, an arm pivotably connected to a front end of the boom, and an arm cylinder which hydraulically causes the arm to pivot; a hydraulic actuator circuit which has a first circuit including the boom cylinder and a boom control valve for controlling an operation of the boom, a second circuit including the arm cylinder and an arm control valve for controlling an operation of the arm cylinder, and a third circuit including the swivel motor and a swivel control valve for controlling an operation of the swivel motor; a first pump which is a hydraulic pressure source of the first circuit; a second pump which is a hydraulic pressure source of the second circuit; a third pump which is a hydraulic pressure source of the third circuit; a merge valve which has a first position and a second position and which forms a fluid path for causing third pump fluid which is hydraulic fluid discharged by the third pump to merge into the first circuit in parallel with the swivel motor at the first position and prohibits the third pump fluid from the merge into the first circuit at the second position; and a merge selecting control section which controls switching of the position of the merge valve, wherein the merge selecting control section holds the merge valve at the first position, when a boom raising operation, which is an operation for moving the boom in a raising direction, is performed during the swivel operation, characterized in that the merge selecting control section holds the merge valve at the first position also when only a swivel operation, which is an operation for the swivel motor, is performed.