A hydroelectric power generator for a river, comprising: height adjusters (61, 62) disposed at a predetermined distance from each other on a concrete base; a lower flow guide unit disposed between side guides of which a front and rear thereof are connected to the height adjusters (61, 62), and having a blade guide at front end of a flow guide; a turbine disposed on a turbine shaft over the lower flow guide unit and having blade seats on an outer side thereof; blades disposed on fixed shafts passing through the turbine in the blade seats and deployed by a stream to rotate the turbine; and an upper flow guide unit disposed between the side guides connected to the height adjuster over the lower flow guide unit and having a power generation guide along which the blades on the outer side of the turbine are deployed and moved, wherein the power generation guide is formed to have a predetermined gap from the outer side of the turbine, and velocity heads are made by a gap between ends of the blades and the power generation guide when the blades are deployed and rotated, and the same pressure is applied to the blades and torque corresponding to the number of the blades is applied to the turbine, and water and the blades move in the same direction to generate electricity by rotating the turbine, and the drain space is the same as or wider than the gap between the turbine and a power generation guide to make the speed and pressure of water that is discharged the same as or larger than the external flow speed and pressure so that water can be easily discharged, wherein the upper flow guide unit has a control gate, which increases rotational energy by increasing an angular speed of the turbine by controlling the amount and flow speed of water supplied to the turbine by controlling an angle through a gate rotary shaft, at a front thereof, and cylinder shafts connected to a gate shaft at an upper front are connected to the turbine shaft through gate control cylinders to control the angle of the control gate. height adjusters (61, 62) disposed at a predetermined distance from each other on a concrete base; a lower flow guide unit disposed between side guides of which a front and rear thereof are connected to the height adjusters (61, 62), and having a blade guide at front end of a flow guide; a turbine disposed on a turbine shaft over the lower flow guide unit and having blade seats on an outer side thereof; blades disposed on fixed shafts passing through the turbine in the blade seats and deployed by a stream to rotate the turbine; and an upper flow guide unit disposed between the side guides connected to the height adjuster over the lower flow guide unit and having a power generation guide along which the blades on the outer side of the turbine are deployed and moved, wherein the power generation guide is formed to have a predetermined gap from the outer side of the turbine, and velocity heads are made by a gap between ends of the blades and the power generation guide when the blades are deployed and rotated, and the same pressure is applied to the blades and torque corresponding to the number of the blades is applied to the turbine, and water and the blades move in the same direction to generate electricity by rotating the turbine, and the drain space is the same as or wider than the gap between the turbine and a power generation guide to make the speed and pressure of water that is discharged the same as or larger than the external flow speed and pressure so that water can be easily discharged, wherein the upper flow guide unit has a control gate, which increases rotational energy by increasing an angular speed of the turbine by controlling the amount and flow speed of water supplied to the turbine by controlling an angle through a gate rotary shaft, at a front thereof, and cylinder shafts connected to a gate shaft at an upper front are connected to the turbine shaft through gate control cylinders to control the angle of the control gate.