A fan motor drive device configured to drive a fan with a motor includes a position speed estimating calculator configured to calculate estimated values of a rotation angle and a rotation speed of the motor, a multiplier configured to calculate a control target value by multiplying the rotation speed calculated in the position speed estimating calculator and a flow coefficient, and a current vector controller configured to control a drive current of the motor by feedback control based on the control target value. A rotor magnetic flux intensity of the motor is calculated based on at least a drive current obtained by a current sensor and the rotation angle obtained from the position speed estimating calculator, and a control value used in driving the motor is changed by the rotor magnetic flux intensity thus calculated.