Abstract: In a paddle-wheel type flow meter: a paddle wheel has its paddle tips exposed to a fluid being measured; a light source emits a light to one of a pair of optical-fiber cables which are oppositely disposed from each other through a paddle-wheel housing so as to provide a transmission path of the light across the housing, the transmission path being intermittently blocked off with the paddle tips of the paddle wheel as the paddle wheel is rotatably driven by thge fluid to produce an optical-pulse train directly proportional to the rotational speed of the paddle wheel; the optical-pulse train is translated into an electrical-pulse train through an optical receiver or photoelectric converter connected to the other of the pair of the optical-fiber cables; and the electrical-pulse train is supplied to a suitable digital instrument to determine a flow rate of the fluid, because the flow rate is substantially proportional to a frequency of the electrical-pulse train corresponding to the rotational speed of the paddle