At a refinery, a leakage and fire occurred during usual operation at a medium-pressure hydrocracker for the desulfurization and decomposition of heavy gas oil. Gas of high-temperature and high-pressure leaked through a flange with a special shape located at the reactor outlet, and a fire occurred. The fire occurred on changing operating conditions of a medium-pressure hydrocracker for gas oil at a refinery. Process fluid leaked from the flange with a spacer at the reactor outlet after the operation temperature of the reactor was lowered, and a fire occurred. There fire damaged the pipeline in the area. The spacer shrank as temperature fell but the bolt did not shrink, and the tightening torque of the flange decreased. Parallelism of the flange was not sufficient, and use of the spacer seemed to be a true cause of the accident.The operating temperature was lowered the day prior to the fire; consequently, the spacer of the flange with the spacer contracted thermally, but the bolt of the flange did not shrink thermally to the same extent. Therefore, tightening torque of the spacer at both ends of the gasket became insufficient, and gas leaked. Although there is no description of the ignition source, it could be static electricity caused due to the spouting material, or gas temperature.