An apparatus for controlling a throttle opening degree of a diesel engine with a supercharger is disclosed. The apparatus includes an electronic control unit having functions of detecting section, a computing section, a correcting section, and a correction limiting/inhibiting section. The detecting section detects the atmospheric pressure, and the computing section computes an atmospheric pressure correction value with respect to the throttle opening degree based on the detected atmospheric pressure. The correcting section corrects the throttle opening degree in accordance with the atmospheric pressure correction value. When the degree of separation of an actual charging pressure from a target charging pressure in the supercharger is les than a predetermined value, the correction limiting/inhibiting section limits or inhibits correction of the throttle opening degree in accordance with the atmospheric pressure correction value.