A malfunction diagnostic apparatus for a combustion improvement device that is disposed in an intake passage connected to each of combustion chambers in an internal combustion engine to improve a combustion state, includes a detector that detects an average crank angular acceleration or a correlated value correlated with the average crank angular acceleration in each of cylinders in the internal combustion engine; an operating state detector that detects a current operating state of the combustion improvement device; and a malfunction determination device that determines whether a malfunction occurs in the combustion improvement device for each of the cylinders, based on the average crank angular acceleration or the correlated value and the current operating state of the combustion improvement device.