Disclosed is an electronic controller for a diesel engine that includes a supply pump (2) for supplying fuel under pressure to a common rail (3) provided on a fuel supply line, the supply pressure level of the supply pump (2) being controllable, and a pressure reducing valve (7) for reducing the pressure of fuel in the common rail (3) (the rail pressure). The controller (10) automatically stops the engine when predetermined automatic stop conditions are satisfied during idling of the engine as well as restarts the engine when predetermined restart conditions are satisfied while the engine is at an automatic standstill. Furthermore, the controller (10) controls the supply pressure level of the supply pump (2) so as to make the rail pressure higher for the engine at an automatic standstill than a target value (PRidle) of the rail pressure during idling. The controller (10) also holds the pressure reducing valve (7) closed while the engine is at an automatic standstill and as well provides control to the degree of opening of the pressure reducing valve (7) so that when the engine is restarted, the rail pressure is at a target pressure (target rail pressure at restart) that is required for a restart.