When an object detected by a distance measuring sensor is located in a monitoring region set in a blind spot of an operator, an obstacle determination section determines the detected object as an obstacle. Then, the obstacle determination section sets a monitoring region so that a region indicating a lower travelling body (2) is excluded according to a slewing angle detected by an angle sensor. Based on the slewing angle detected by the angle sensor, a stop control section determines a component, of at least one of the lower travelling body and an upper slewing body, which has a possibility that a construction machine collides with the obstacle when the component is operated, and stops operation of the determined component.