A device and method for calculating the work load of an engine more efficiently and accurately. The device for calculating the work load of an engine comprises a means for determining a desired frequency component to calculate the work load of the engine obtained through frequency decomposition of the volume variation rate of the engine. The device further comprises a first calculating means for calculating a first correlation coefficient between the volume variation rate and a reference signal composed of a frequency corresponding to the determined component, and a second calculating means for calculating a second correlation coefficient between the cylinder inner pressure of the engine and a reference signal composed of a frequency corresponding to the determined component. The work load of the engine is calculated using the first and second correlation coefficients.