To provide a safety device for conducting the rear obstacle detecting control of a walking type working machine for the purpose of protecting an operator by detecting an obstacle in the rear of the operator without detecting the operator.

An obstacle sensor 2 is a sensor for detecting an object and distance information to the object in non-contact state in the air, and having a lateral detecting range from the width of the human body to the width of the working machine both inclusive. Plural sensors are disposed toward the operator, with at least two sensors sandwiching the operator, thereby detecting an object in the rear of the operator during reverse movement of the moving machine. Further, according to the distance to the obstacle, a warning is given or traveling is automatically stopped.