The purpose of the present invention is: to enable hot water at a required temperature to be supplied, even in cases when compressor load factor is low; to suppress heat dissipation from a waste-heat recovery device; and to improve waste-heat recovery rate. A waste-heat recovery system in an oil-cooled gas compressor is provided with: a compressor main body (3); an oil separator (6); gas piping (8) which supplies, to a demanded destination, compressed gas separated from oil by the oil separator; oil piping (7) which returns, to the compressor main body, the oil separated by the oil separator; and a waste-heat-recovery heat exchanger (10) which recovers heat from the compressed gas and/or the oil. The waste-heat recovery system is also provided with: a hot-water storage tank (19); circulation circuits (17, 18) in which a heat medium is made to circulate between the waste-heat recovery heat exchanger and the hot-water storage tank; a circulation pump (22) provided to these circulation circuits; and a control device (32) which, in cases when the temperature of the oil or the compressed gas subjected to heat exchange in the waste-heat-recovery heat exchanger is equal to or less than the temperature of the hot water in the hot-water storage tank, stops the circulation pump or reduces the rotational frequency of the circulation pump.