A electric power control device for a hybrid electric power system comprising: an electric power measuring unit that is configured to measure output electric power of the electric power system; and a controller that is configured to control operations of a first electric power generator and a second electric power generator of the electric power system based on a preset control mode and the measured output electric power, characterized in that the controller is configured to control an output voltage of the first electric power generator to be changed by a first offset voltage corresponding to an output of the second electric power generator for a first time period when the output of the second electric power generator is controlled to be switched from an ON state into an OFF state, wherein the first offset voltage is a voltage for offsetting that the output voltage of the second electric power generator is gradually changed to zero when switched from the ON state to the OFF state, wherein the controller is configured to control an output voltage of the first electric power generator to be changed by a second offset voltage corresponding to an output of the second electric power generator for a second time period when the output of the second electric power generator is controlled to be switched from the OFF state into the ON state, wherein the second offset voltage is a voltage for offsetting that the output voltage of the second electric power generator is gradually increasing when switched from the OFF state to the ON state, and wherein a response rate of an output change by the control of the first electric power generator is configured to be faster than a response rate of an output change by the control of the second electric power generator. an electric power measuring unit that is configured to measure output electric power of the electric power system; and a controller that is configured to control operations of a first electric power generator and a second electric power generator of the electric power system based on a preset control mode and the measured output electric power, characterized in that the controller is configured to control an output voltage of the first electric power generator to be changed by a first offset voltage corresponding to an output of the second electric power generator for a first time period when the output of the second electric power generator is controlled to be switched from an ON state into an OFF state, wherein the first offset voltage is a voltage for offsetting that the output voltage of the second electric power generator is gradually changed to zero when switched from the ON state to the OFF state, wherein the controller is configured to control an output voltage of the first electric power generator to be changed by a second offset voltage corresponding to an output of the second electric power generator for a second time period when the output of the second electric power generator is controlled to be switched from the OFF state into the ON state, wherein the second offset voltage is a voltage for offsetting that the output voltage of the second electric power generator is gradually increasing when switched from the OFF state to the ON state, and wherein a response rate of an output change by the control of the first electric power generator is configured to be faster than a response rate of an output change by the control of the second electric power generator.