An electronic controller for a diesel engine (1) performs primary injection control in which primary injection of fuel is controlled based on an operational status of the diesel engine and additional injection control in which additional injection of the fuel is controlled for estimation of a cetane number of the fuel. The electronic controller includes a control means that, as the additional injection control, causes a plurality of fuel injections to be performed at different injection timings as the additional injection, calculates the amount of increase in torque of a crankshaft (14) due to each of the fuel injections, estimates injection timing at which misfiring starts to occur based on a trend of variation in the calculated torque increase amount as the injection timing of the fuel injections is shifted in one direction, and estimates the cetane number of the fuel based on the estimated injection timing.