A steering system for an engineering vehicle comprising: a hydraulic pump; a steering cylinder (103a, 103b) driven by pressurized oil discharged from the hydraulic pump; a steering valve for controlling the direction and flow rate of the pressurized oil supplied from the hydraulic pump to the steering cylinder; and a steering wheel manipulated by an operator, said steering system switching the steering valve based on the rotational direction and rotational quantity of the steering wheel so as to control the steering cylinder (103a, 103b), wherein said steering system further includes: a priority valve that is located between the hydraulic pump and the steering valve and that includes setting means (24c, 25) for setting a target value of the differential pressure across the steering valve, said priority valve supplying by priority the steering cylinder (103a, 103b) with the pressurized oil discharged from the hydraulic pump and supplying an operational actuator (113, 114) for driving an operating mechanism with a surplus flow of the pressurized oil, by controlling the differential pressure across the steering valve so that the differential pressure is kept at the target value; pressure detection means for detecting the load pressure of the steering cylinder (103a, 103b); and control means (32, 32A, 33, 29, 24c) for modifying, on the basis of the load pressure detected by the pressure detection means, the target value set by the setting means of the priority valve characterized in that, said control means (32, 32A, 33, 29, 24c) modifies said target value such that when the load pressure is lower than a predetermined value (Pr), the target value is a constant value, and when the load pressure exceeds the predetermined value (Pr), the target value becomes smaller than the constant value. a hydraulic pump; a steering cylinder (103a, 103b) driven by pressurized oil discharged from the hydraulic pump; a steering valve for controlling the direction and flow rate of the pressurized oil supplied from the hydraulic pump to the steering cylinder; and a steering wheel manipulated by an operator, said steering system switching the steering valve based on the rotational direction and rotational quantity of the steering wheel so as to control the steering cylinder (103a, 103b), wherein said steering system further includes: a priority valve that is located between the hydraulic pump and the steering valve and that includes setting means (24c, 25) for setting a target value of the differential pressure across the steering valve, said priority valve supplying by priority the steering cylinder (103a, 103b) with the pressurized oil discharged from the hydraulic pump and supplying an operational actuator (113, 114) for driving an operating mechanism with a surplus flow of the pressurized oil, by controlling the differential pressure across the steering valve so that the differential pressure is kept at the target value; pressure detection means for detecting the load pressure of the steering cylinder (103a, 103b); and control means (32, 32A, 33, 29, 24c) for modifying, on the basis of the load pressure detected by the pressure detection means, the target value set by the setting means of the priority valve characterized in that, a priority valve that is located between the hydraulic pump and the steering valve and that includes setting means (24c, 25) for setting a target value of the differential pressure across the steering valve, said priority valve supplying by priority the steering cylinder (103a, 103b) with the pressurized oil discharged from the hydraulic pump and supplying an operational actuator (113, 114) for driving an operating mechanism with a surplus flow of the pressurized oil, by controlling the differential pressure across the steering valve so that the differential pressure is kept at the target value; pressure detection means for detecting the load pressure of the steering cylinder (103a, 103b); and control means (32, 32A, 33, 29, 24c) for modifying, on the basis of the load pressure detected by the pressure detection means, the target value set by the setting means of the priority valve characterized in that, said control means (32, 32A, 33, 29, 24c) modifies said target value such that when the load pressure is lower than a predetermined value (Pr), the target value is a constant value, and when the load pressure exceeds the predetermined value (Pr), the target value becomes smaller than the constant value.