A relief pressure switching apparatus for hydraulic working machine, comprising: a hydraulic actuator circuit, a relief pressure of the hydraulic actuator circuit being switched in accordance with the type of a working apparatus (3; 4) to be installed, a compression crushing apparatus of an openable and closable type and a breaker of an oscillating type being selectively installed as the working apparatus (3; 4); a hydraulic pump serving as an actuator hydraulic source of a hydraulic actuator (8, 9) of the working apparatus (3; 4); a pump regulator for controlling a discharge flow rate of the hydraulic pump; a controller for storing a plurality of relief pressures preliminarily set in combination with a flow rate of oil, and commanding flow rates and a relief pressure selected from stored combinations between the flow rates and the relief pressures as matching the working apparatus (3; 4) to said pump regulator and said relief valves (19, 20), and characterized by electromagnetically variable relief valves (19, 20) for determining the relief pressure; wherein said controller is adapted to be switched between a setting mode for setting a plurality of combinations between the flow rates and the relief pressures for each of the compression crushing apparatus and the breaker and a selection mode for selecting a flow rate and a relief pressure for the installed working apparatus (3; 4) from the plurality of combinations between the flow rates and the relief pressures set in the setting mode, and in the setting mode, by the controller, the relief pressure is set on the basis of a pressure generated when the hydraulic actuator (8, 9) is brought into action by the flow rate to be combined for the working apparatus (3; 4) to be installed, and said controller commands the flow rate and the relief pressure selected in the selection mode to said pump regulator and said relief valves (19, 20). a hydraulic actuator circuit, a relief pressure of the hydraulic actuator circuit being switched in accordance with the type of a working apparatus (3; 4) to be installed, a compression crushing apparatus of an openable and closable type and a breaker of an oscillating type being selectively installed as the working apparatus (3; 4); a hydraulic pump serving as an actuator hydraulic source of a hydraulic actuator (8, 9) of the working apparatus (3; 4); a pump regulator for controlling a discharge flow rate of the hydraulic pump; a controller for storing a plurality of relief pressures preliminarily set in combination with a flow rate of oil, and commanding flow rates and a relief pressure selected from stored combinations between the flow rates and the relief pressures as matching the working apparatus (3; 4) to said pump regulator and said relief valves (19, 20), and characterized by electromagnetically variable relief valves (19, 20) for determining the relief pressure; wherein said controller is adapted to be switched between a setting mode for setting a plurality of combinations between the flow rates and the relief pressures for each of the compression crushing apparatus and the breaker and a selection mode for selecting a flow rate and a relief pressure for the installed working apparatus (3; 4) from the plurality of combinations between the flow rates and the relief pressures set in the setting mode, and in the setting mode, by the controller, the relief pressure is set on the basis of a pressure generated when the hydraulic actuator (8, 9) is brought into action by the flow rate to be combined for the working apparatus (3; 4) to be installed, and said controller commands the flow rate and the relief pressure selected in the selection mode to said pump regulator and said relief valves (19, 20).