To reduce noise caused by impact generated during spallation performed by using a hydraulic breaker without processing a breaker body and covering the breaker body with a soundproof cover with simple means.

The means comprise steps of: detecting hydraulic pressure rise of a piston which impacts on a chisel during performing spallation work of a hydraulic breaker; creating imitation sound which has reverse phase to phase of impact sound generated when the piston impacts on the chisel and recorded previously immediately before the piston impacts on the chisel and emitting the imitation sound toward whole directions in the vicinity of the chisel at the same time to impact; and overlapping the impact sound and the imitation sound. Thereby, noise generated around the breaker is reduced.