Fuel injection amount calculation method and fuel injection controlling apparatus

To provide a fuel injection amount calculation method and a fuel injection controlling apparatus wherein, even if a displacement appears at opening and closing timings of valves due to an assembly error or time-dependent variation of the tappet clearance, a suitable fuel injection amount can be calculated to achieve improvement in fuel cost and purification of exhaust gas. The fuel injection amount calculation method for calculating a fuel injection amount to an internal combustion engine (2) of a vehicle includes calculating a relative intake pressure which is a difference between an intake pressure peak of intake air upon intake starting of a cylinder of the internal combustion engine and an intake pressure bottom of the intake air upon intake ending, and calculating the fuel injection amount based on the relative intake pressure.