A brushless motor drive device configured to drive a brushless motor, the brushless motor drive device comprising: an inverter circuit configured to energize and drive a winding of the brushless motor; a current detection circuit configured to detect a current value of the winding; a control unit configured to control rotation of the brushless motor; and a resistor-capacitor filter including a resistor and a capacitor, characterized in that the resistor and the capacitor being connected in series, wherein the control unit includes: a drive control unit configured to generate a signal to drive the inverter circuit; a clock generation circuit configured to generate a clock pulse to be used as a reference for an operation period; a pulse output circuit configured to generate a pulse signal with a changing frequency based on the clock pulse and to apply the pulse signal to the resistor-capacitor filter, the changing frequency is changed in accordance with a pulse width (Th1, Th2, Th3) of a high period (Th), wherein a duty ratio defined as the time width of the high period (Th) to the time width of a corresponding cycle period (Tt) is constant; an analog-digital converter circuit connected to a connection point between the resistor and the capacitor and the current detection circuit; and an AD-conversion-error calculation unit configured to calculate a conversion error of the analog-digital converter circuit, the AD-conversion-error calculation unit calculating the conversion error based on a difference between an output value of the analog-digital converter circuit produced in response to an input of a voltage of the capacitor and an analog-digital-converted value calculated from a charging time of the capacitor. an inverter circuit configured to energize and drive a winding of the brushless motor; a current detection circuit configured to detect a current value of the winding; a control unit configured to control rotation of the brushless motor; and a resistor-capacitor filter including a resistor and a capacitor, characterized in that the resistor and the capacitor being connected in series, wherein the control unit includes: a drive control unit configured to generate a signal to drive the inverter circuit; a clock generation circuit configured to generate a clock pulse to be used as a reference for an operation period; a pulse output circuit configured to generate a pulse signal with a changing frequency based on the clock pulse and to apply the pulse signal to the resistor-capacitor filter, the changing frequency is changed in accordance with a pulse width (Th1, Th2, Th3) of a high period (Th), wherein a duty ratio defined as the time width of the high period (Th) to the time width of a corresponding cycle period (Tt) is constant; an analog-digital converter circuit connected to a connection point between the resistor and the capacitor and the current detection circuit; and an AD-conversion-error calculation unit configured to calculate a conversion error of the analog-digital converter circuit, the AD-conversion-error calculation unit calculating the conversion error based on a difference between an output value of the analog-digital converter circuit produced in response to an input of a voltage of the capacitor and an analog-digital-converted value calculated from a charging time of the capacitor. a drive control unit configured to generate a signal to drive the inverter circuit; a clock generation circuit configured to generate a clock pulse to be used as a reference for an operation period; a pulse output circuit configured to generate a pulse signal with a changing frequency based on the clock pulse and to apply the pulse signal to the resistor-capacitor filter, the changing frequency is changed in accordance with a pulse width (Th1, Th2, Th3) of a high period (Th), wherein a duty ratio defined as the time width of the high period (Th) to the time width of a corresponding cycle period (Tt) is constant; an analog-digital converter circuit connected to a connection point between the resistor and the capacitor and the current detection circuit; and an AD-conversion-error calculation unit configured to calculate a conversion error of the analog-digital converter circuit, the AD-conversion-error calculation unit calculating the conversion error based on a difference between an output value of the analog-digital converter circuit produced in response to an input of a voltage of the capacitor and an analog-digital-converted value calculated from a charging time of the capacitor.