PURPOSE: To minimize increase in noise even when a motor is attached on a suction side by specifying a ration of an outside of the motor to the inside diameter of a suction port of a casing and also a ratio of an outside diameter of an impeller to that of the motor.

CONSTITUTION: A motor 13 is arranged within a casing of a mixed flow fan through a stay 11a. A mixed flow impeller 14 is attached to the shaft of the motor 13. By changing an outside diameter A of the impeller 4, an inside diameter B of a suction port in the casing 11 and an outside diameter C of a motor, the change in noise against a ratio of the inside diameter B of the suction port to the outside diameter C of the motor is examined, which gives the following result. With the ratio of B to C being in a range from 0.2 to 0.45, and with the ration of C to A being in a range from 0.2 to 0.3, the noise value is made at minimum. Accordingly, by setting the dimensions as mentioned above, it is possible to obtain the mixed flow fan of suction side motor attaching type for a tone quality heater capable of being reduced in noise at a low cost.