PURPOSE: To provide a device for restraining and eliminating air bubbles in a fossil fuel activator which performs the power-up and exhaust emission control of a diesel engine.

CONSTITUTION: A fossil fuel active device 4 is installed in a fuel feeding pipe 3 lying between an injection pump 11 and a fuel tank 2 of a diesel engine 1. A solenoid pump 5 is installed at the upstream side of this fossil fuel active device 4 and/or an air pressurizing pump 51 is attached to the fuel tank 2. A defoaming pipe 31 is installed in space between an outlet side upper part of the fossil fuel active device 4 and the fuel tank 2 at need. Fuel oil is fed under pressure doing like that, air bubbles in the fossil fuel active device 4 is prevented from occurring, while the produced air bubbles are circulated around the fuel tank 2 through the defoaming pipe 31 together with a part, of the fuel oil. The solenoid pump is installed in this defoaming pipe 31, through which the air bubbles can be drawn and eliminated if necessary.