A hydroelectric power generation system comprising: a hydraulic turbine disposed in a channel through which a fluid flows; a generator driven by the hydraulic turbine ; a head adjuster adjusting an effective head of the hydraulic turbine ; and a controller cooperatively executing: flow rate control for controlling the generator such that a flow rate in the hydraulic turbine is brought close to a target flow rate; and head adjusting control for adjusting the effective head of the hydraulic turbine using the head adjuster such that the effective head of the hydraulic turbine falls within a first range, characterised in that if a condition indicating that the effective head of the hydraulic turbine is larger than an upper limit value of the first range is satisfied, the controller executes, as the head adjusting control, first control for reducing the effective head of the hydraulic turbine using the head adjuster , wherein the upper limit value of the first range is a determination value for the determining whether or not the operation point of the hydraulic turbine has reached the cavitation region and characterised in that if a condition indicating that the effective head of the hydraulic turbine is smaller than a lower limit value of the first range is satisfied, the controller executes, as the head adjusting control, second control for increasing the effective head of the hydraulic turbine using the head adjuster , the lower limit value of the first range is a determination value for determining whether or not the hydraulic turbine has reached the operating limit curve, and the operating limit curve corresponds to an operation point at which it becomes impossible to adjust the flow rate in the hydraulic turbine. a hydraulic turbine disposed in a channel through which a fluid flows; a generator driven by the hydraulic turbine ; a head adjuster adjusting an effective head of the hydraulic turbine ; and a controller cooperatively executing: flow rate control for controlling the generator such that a flow rate in the hydraulic turbine is brought close to a target flow rate; and head adjusting control for adjusting the effective head of the hydraulic turbine using the head adjuster such that the effective head of the hydraulic turbine falls within a first range, characterised in that if a condition indicating that the effective head of the hydraulic turbine is larger than an upper limit value of the first range is satisfied, the controller executes, as the head adjusting control, first control for reducing the effective head of the hydraulic turbine using the head adjuster , wherein the upper limit value of the first range is a determination value for the determining whether or not the operation point of the hydraulic turbine has reached the cavitation region and characterised in that if a condition indicating that the effective head of the hydraulic turbine is smaller than a lower limit value of the first range is satisfied, the controller executes, as the head adjusting control, second control for increasing the effective head of the hydraulic turbine using the head adjuster , the lower limit value of the first range is a determination value for determining whether or not the hydraulic turbine has reached the operating limit curve, and the operating limit curve corresponds to an operation point at which it becomes impossible to adjust the flow rate in the hydraulic turbine. <EOS>