A hydraulic circuit system for a hydraulic excavator, which has a hydraulic circuit for an attachment, includes: a directional control valve 16 that supplies a hydraulic fluid to an attachment connected to the hydraulic circuit for an attachment; a first over-load relief valve unit 38 that is provided in one of lines of the hydraulic circuit for an attachment; a second over-load relief valve unit 39 that is provided in the other of the lines of the hydraulic circuit for an attachment; a first proportional solenoid valve 40 that controls set relief pressure of the first over-load relief valve unit 38; and a second proportional solenoid valve 41 that controls set relief pressure of the second over-load relief valve unit 39. The attachment having various characteristics and capacity, such as a hydraulic breaker or a crusher, can be applied to the hydraulic circuit for an attachment in a simple configuration.