To sufficiently control a power generation frequency, to achieve extremely good recovery of surplus exhaust gas energy, to control the rotational speed of a hydraulic pump or generators without being markedly affected by the operation of a supercharger, and to achieve the high degree of freedom in system design.

This rotational speed control system includes: a diesel engine 1 or a gas engine having a supercharger 5; a power turbine 10 rotatively driven by introducing exhaust gas from the engine from the upstream side of the supercharger; the hydraulic pump 12 rotatively driven by the power turbine; the first generator 15 rotatively driven by the hydraulic pump; and a hydraulic pump load control means 12 controlling a load on the hydraulic pump. The pump load control means controls the operation of the hydraulic pump so that the rotational speed of the power turbine is substantially kept constant. The pump load control means is composed of a variable displacement hydraulic pump and a controller adjusting the load on the hydraulic pump based on the power generation frequency of the first generator.