To provide an engine speed control device capable of carrying out PTO work while keeping engine rotation constant; and its method.

This engine speed control device can keep the engine speed at a constant value when the power of the engine is extracted to the outside in an automobile 1 having a power take-off mechanism 3. In the engine speed control device, the automobile 1 has an auto-cruise mechanism keeping a traveling speed of the automobile constant without operating an accelerator, and is structured to execute control of keeping the engine speed constant without changing it by operating the auto-cruise mechanism when the power of the engine 1 is extracted to the outside by operating the power take-off mechanism 3.

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