To provide a parallel arrangement of constant-pressure fluid-conveying devices for energy saving.

The parallel arrangement of constant-pressure fluid-conveying devices for energy saving can flexibly supply a large amount of gaseous matter and water to be needed to each production unit under a constant-pressure condition. The device includes a centrifugal fluid conveying machine having a frequency conversion function, power meter, barometer, flowmeter, and controller. A performance curve of the parallel arrangement of the devices is set inside the controller together with a result of theoretic analysis of data such as a system impedance curve on a load side and an equivalent curve data provided from a manufacturer. Upon operation, by comparing the data set inside the controller with an actual measurement obtained by the attached barometer, power meter, and flowmeter, the optimum energy saving condition is obtained. This condition is made to be an operational rule of the devices.