A method of estimating an air-intake amount of an internal combustion engine is provided. The method may comprise detecting a fluid energy amount in an interior of an air-intake passage at first and second points in time while an intake valve is closed from a compression stroke to an exhaust stroke, and calculating a predicted air-intake amount using the values of the fluid energy amounts at the first and second points with reference to an air-intake amount calculation map showing a correlation between the values of the fluid energy amounts at the first and second points and the predicted air-intake amount in the intake stroke, the air-intake amount calculation map being pre-created by finding the values of the fluid energy amounts in the air-intake passage at the first and second points and the air-intake amount, for plural running states of the internal combustion engine.