An air flow velocity calculating device of a internal combustion engine, has a cylinder, a spark plug, an air intake passage, an air flow controlling valve, an air flow velocity calculating portion, and an electric discharge current controlling portion. An electric discharge between a center electrode and a ground electrode is sustained during an electric discharge sustaining period. The electric discharge sustaining period is a period from when the electric discharge begins to when the electric discharge is intercepted by the air flow. Energy is supplied to the spark plug, so that the electric discharge current is sustained at a constant value by the electric discharge current controlling portion during an ignition period. The ignition period is a sum of an electric discharge sustaining period of a first electric discharge and electric discharge sustaining periods of second and subsequent electric discharges in one cycle of the internal combustion engine.