PURPOSE:To prevent the occurrence of turbulent flow in the upstream and downstream of the parallel fluid channel accompanying the switch-over of the channel by providing a fluid flow adjusting mechanism in series in the parallel fluid channel consisting of the main channel with a turbine and of a side channel with a channel opening-closing mechanism. CONSTITUTION:Natural gas supplied from a supply source 1 is branched and introduced into the main channel 2 and the side channel 7 via a flow-rate adjusting mechanism 9. So as not to cause turbulence in flow even when the channel is switched from the main channel 2 to the side channel 7, the characteristics of pressure and flow rate of the main and side channels 2 and 7 are required to be equal before and after switching. Thus, the flow-rate adjusting mechanism 3 of the main channel 2 is held in the state of being completely opened only by adjustment of flow rate by the flow-rate adjusting mechanism 9, whereby the characteristics of pressure and flow rate of the main channel 2 becomes constant and adjustment of the characteristics of pressure and flow rate of the side channel 7 becomes unnecessary.