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. The water jet peening device is provided with: a guiding part (10) having a tip part (10a) disposed in a position aligned with a predetermined distance over which the water jet is sprayed from the spray port, the guiding part being capable of moving with the spray nozzle along a movement trajectory; a pressing movement mechanism (9) for pressing on and moving the spray nozzle and the guiding part along the direction in which the water jet is sprayed from the spray port; a nozzle position detection means (15e) for detecting the movement position of the spray port in the movement trajectory; and a control means (20) for controlling the pressing force of the pressing movement mechanism on the guiding part, on the basis of the movement position of the spray port detected by the nozzle position detection means.