The abutment of the rod to the work piece is accomplished in a stable manner by providing an air passage in the inside of the work support for supplying pressurized air for detecting the abutment between the rod and the work piece. Work support is equipped with case, rod, sleeve that elastically deforms to reduce its diameter, hydraulic cylinder for driving rod in an advancing direction, spring for elastically energizing rod in advancing direction, detection nozzle for ejecting pressurized air, ...