PURPOSE: To easily detect an abnormal condition when this abnormal condition, in which no combustion is generated in a certain cylinder, is provided, in a multicylinder engine having an electronic control type fuel injection device.

CONSTITUTION: In a trouble diagnosing method for detecting abnormality in a fuel injection system of a multicylinder internal combustion engine, a rotational pulse signal of a crankshaft is detected, to obtain a pulse time interval at combustion time by fuel injection against each cylinder. A deviation between this pulse time interval and a pulse time interval of the other cylinder is calculated to compare this deviation with a reference value, and abnormality of a cylinder is discriminated. A deviation between a pulse time interval of an aiming at cylinder and a pulse time interval of the preceding combustion cylinder and a deviation between the pulse time interval of the aiming at cylinder and a pulse time interval of the following combustion cylinder are calculated, and by comparing both the deviations with a permissible value, an abnormality cylinder is specifically discriminated.