PURPOSE: To reduce the width in the forward/backward direction of a gear case so as to accomplish compactification by storing at least front end part of flyweight within the width in the forward/backward direction of a flange part and arranging a governor pivot, a governor lever, and the like in the further rear side beyond the flyweight.

CONSTITUTION: A case mounting hole 14, into which a boss 15 projecting to the front side from an injection pump case 6 is fitted, is perforated in a flange part 5 extended in the lateral direction of a cylinder block 3, and a flyweight 16 of a governor mechanism 9 is installed in a fuel injection can shaft 8 penetrating the inside of the boss 15. At least front side part 17 of the flyweight 16 is stored within the width 18 in the forward/backward direction of the flange part 5. Between the fuel injection pump 7 and the flyweight 16, a governor lever pivot 19 is arranged in the lateral direction, and between a governor lever 20 journalled to the pivot 19 and the flyweight 16, a governor sleeve 23 is fitted around the fuel injection can shaft 8. The governor lever is arranged so as to be led outward to the lateral side of the fuel injection pump 7.