A variable-speed operation control apparatus of a hydroelectric power generation system including a power generator interconnected to a power system including a main transformer through a main circuit and a water turbine directly connected to the power generator, the variable-speed operation control apparatus comprising: a branch circuit branched from the main circuit and including a frequency converter which performs conversion between an output frequency of the power generator and a system frequency of the power system, wherein the frequency converter has a smaller capacity in comparison to the power generator, characterized in that the main circuit comprises a current transformer and a potential transformer, respectively configured to detect a current and a voltage when the main circuit is used, the branch circuit comprises a current transformer and a potential transformer, respectively configured to detect a current and a voltage when the branch circuit is used, the branch circuit further comprising a second breaker and the main circuit comprises a first breaker, the second breaker is connected between the power generator and the frequency converter, and the first breaker is directly connected between the power generator and the main transformer to pass an output from the power generator to the main transformer, and the variable-speed operation control apparatus further comprises a controller configured to switch an interconnection route to interconnect the power generator with the power system through the branch circuit and reducing a rotation speed of the power generator and the water turbine by operating the frequency converter, and to determine whether to switch the route interconnecting the power generator to the power system P, based on obtained information and calculated information, wherein the obtained information comprises a water level, a water flow rate, the current and voltage detected by the current transformer and the potential transformer in the main circuit, and the current and voltage detected by the current transformer and the potential transformer in the branch circuit, wherein the calculated information comprises an output power of the power generator, a frequency-converted output power of the power generator and a drop from the water level. a branch circuit branched from the main circuit and including a frequency converter which performs conversion between an output frequency of the power generator and a system frequency of the power system, wherein the frequency converter has a smaller capacity in comparison to the power generator, characterized in that the main circuit comprises a current transformer and a potential transformer, respectively configured to detect a current and a voltage when the main circuit is used, the branch circuit comprises a current transformer and a potential transformer, respectively configured to detect a current and a voltage when the branch circuit is used, the branch circuit further comprising a second breaker and the main circuit comprises a first breaker, the second breaker is connected between the power generator and the frequency converter, and the first breaker is directly connected between the power generator and the main transformer to pass an output from the power generator to the main transformer, and the variable-speed operation control apparatus further comprises a controller configured to switch an interconnection route to interconnect the power generator with the power system through the branch circuit and reducing a rotation speed of the power generator and the water turbine by operating the frequency converter, and to determine whether to switch the route interconnecting the power generator to the power system P, based on obtained information and calculated information, wherein the obtained information comprises a water level, a water flow rate, the current and voltage detected by the current transformer and the potential transformer in the main circuit, and the current and voltage detected by the current transformer and the potential transformer in the branch circuit, wherein the calculated information comprises an output power of the power generator, a frequency-converted output power of the power generator and a drop from the water level. switch an interconnection route to interconnect the power generator with the power system through the branch circuit and reducing a rotation speed of the power generator and the water turbine by operating the frequency converter, and to determine whether to switch the route interconnecting the power generator to the power system P, based on obtained information and calculated information, wherein the obtained information comprises a water level, a water flow rate, the current and voltage detected by the current transformer and the potential transformer in the main circuit, and the current and voltage detected by the current transformer and the potential transformer in the branch circuit, wherein the calculated information comprises an output power of the power generator, a frequency-converted output power of the power generator and a drop from the water level.