PURPOSE: To exchange hydraulic oil at proper time by measuring engine operating time and breaker operating time respectively, and calculating the ratio of them, and displaying this ratio.

CONSTITUTION: A breaker oil pressure detector 3 is installed in the hydraulic oil pressure circuit of a hydraulic breaker, and when breaker oil pressure rises, it inputs a prescribed current to a time count circuit 4, and the time count circuit 4 integrates and stores the time during which the current is inputted similarly to the case of engine oil pressure, and this integrated value is always inputted to a hydraulic breaker operation rate calculation circuit 5. Then, the hydraulic breaker operation rate is calculated by the hydraulic breaker operation rate calculation circuit 5, and a calculated result is inputted to a display device 6 and is displayed. Thus, the latest hydraulic breaker operation rate is displayed on the display device 7, and an operator can know the correct hydraulic oil exchanging time from the displayed breaker operation rate and a graph.