A pulsation detection unit detects, as an actual characteristic, a pulsation frequency of a pressure pulsation, which is caused in fuel pressure, or a physical quantity corresponding to the pulsation frequency, according to a sensor signal from a pressure sensor. The pressure sensor detects a fuel pressure of fuel supplied to a fuel injection valve, which is equipped to an internal combustion engine. A storage unit stores, as a reference characteristic, a reference frequency of a predetermined reference pulsation or a physical quantity corresponding to the reference frequency. A density detection unit detects a fuel density according to a quantity of deviation between the reference characteristic, which is stored in the storage unit, and the actual characteristic, which is detected with the pulsation detection unit.