A water turbine device comprising: a platform provided on a waterway; a suspension support rod for suspending at one end a water turbine that can be immersed in a flowing water in the waterway, an intermediate portion of the suspension support rod is pivotally attached to the platform with a horizontal shaft; a power generation device as a balance weight, the power generation device including a case fixed to the other end of the suspension support rod and a power generator accommodated in the case; and a fixing means for fixing the water turbine to the platform, characterized in that the platform is equipped with a pair of left side and right side rods (4, 4) provided on side walls (1a, 1a) in parallel with the waterway and a plurality of horizontal rods (5, 5) bridged over the side rods (4, 4), wherein an intermediate portion in the vertical direction of the suspension rod is pivotally supported to an intermediate portion between the left and right sides of the front horizontal rod by a bracket and the horizontal shaft oriented in the horizontal direction, wherein the fixing means is provided on the front horizontal rod near the bracket, wherein the water turbine comprises a water-tight case, a water turbine shaft pivoted rotatably by front and rear bearings (11, 12) in the watertight case and a front end of the water turbine shaft is projected outward and forward from the front end of the watertight case in a watertight manner, and an impeller is fixed to the front end of the water turbine shaft in front of the watertight case, wherein the water turbine is configured to be rotatable around the horizontal shaft as a center between a use position where the water turbine is immersed in a water flow in the waterway and a nonuse position above the water flow in the waterway, and wherein the product of the total mass of the balance weight and the distance from the horizontal shaft to the power generation device and the product of the mass of the entire water turbine and the distance from the horizontal shaft to the water turbine are set substantially equal. a platform provided on a waterway; a suspension support rod for suspending at one end a water turbine that can be immersed in a flowing water in the waterway, an intermediate portion of the suspension support rod is pivotally attached to the platform with a horizontal shaft; a power generation device as a balance weight, the power generation device including a case fixed to the other end of the suspension support rod and a power generator accommodated in the case; and a fixing means for fixing the water turbine to the platform, characterized in that the platform is equipped with a pair of left side and right side rods (4, 4) provided on side walls (1a, 1a) in parallel with the waterway and a plurality of horizontal rods (5, 5) bridged over the side rods (4, 4), wherein an intermediate portion in the vertical direction of the suspension rod is pivotally supported to an intermediate portion between the left and right sides of the front horizontal rod by a bracket and the horizontal shaft oriented in the horizontal direction, wherein the fixing means is provided on the front horizontal rod near the bracket, wherein the water turbine comprises a water-tight case, a water turbine shaft pivoted rotatably by front and rear bearings (11, 12) in the watertight case and a front end of the water turbine shaft is projected outward and forward from the front end of the watertight case in a watertight manner, and an impeller is fixed to the front end of the water turbine shaft in front of the watertight case, wherein the water turbine is configured to be rotatable around the horizontal shaft as a center between a use position where the water turbine is immersed in a water flow in the waterway and a nonuse position above the water flow in the waterway, and wherein the product of the total mass of the balance weight and the distance from the horizontal shaft to the power generation device and the product of the mass of the entire water turbine and the distance from the horizontal shaft to the water turbine are set substantially equal.