To enable application of a large capacity motor-driven blower to a site, such as a tunnel in a mountain, etc., where power supply availability is not satisfactory by a method where the starting current of the motor-driven blower is substantially suppressed and a required power supply capacity is reduced.

In a motor-driven blower, whose motor has a star-delta starter, a thyristor voltage varying device 2, which lowers the voltages supplied to power supply lines R, S, and T at starting, is connected in parallel with contactors A, which open/close the power supply lines R, S, and T. At the starting, when an operation signal is turned on, stator windings 11, 12, and 13 are star-connected with each other and the contactors A are kept open. A low-voltage power is supplied to the stator windings 11, 12, and 13 from the thyristor voltage varying device 2, to carry out the star-delta start in a state of reduced operation currents.