To provide a device for preventing a cylinder hydraulic pressure loss of a hydraulic breaker.

A device for preventing a cylinder hydraulic pressure loss of a hydraulic breaker is configured such that sealing grooves are formed on a high pressure valve hole formed on a valve sleeve and upper and lower sides of a low pressure return hole and a high pressure introduction hole at an outer diameter of the valve sleeve in a gap to contact surfaces with a cylinder. A back-up ring blocking high-pressure oil leakage, an O-ring, and a back-up ring are sequentially seated inside the respective sealing grooves formed on the high pressure valve hole and the upper and lower sides of the low pressure return hole and the high pressure introduction hole.