According to the present invention, the distance between a spray nozzle and a surface to be constructed is kept constant, and the spray nozzle is moved smoothly to perform water jet peening in a suitable state of construction. A water jet peening device has a spray nozzle (8) for spraying a water jet, the spray nozzle being capable of moving along a predetermined movement trajectory so that a spray port (8a) of the spray nozzle points upward or downward.