To appropriately control a negative ion supply amount in order to obtain a desired engine output.

This negative ion supply control device for controlling the negative ion supply amount V supplied to an intake passage of an internal combustion engine detects a combustion state in a cylinder, and feedback controls the negative ion supply amount so that the detected combustion state approaches a target combustion state. Concretely, the negative ion supply amount V is feedback controlled based on a deviation max between a predetermined target crank angle maxtrg and a crank angle max that cylinder inner pressure Pc detected by a cylinder inner pressure sensor is maximized.