A motor assembly with a speed reducer comprising: a casing divided into two parts including a box-shaped motor housing having a first opening section formed in one surface thereof, and a housing cover configured to cover the first opening section; a motor received in the motor housing; a speed reducer received in the motor housing and that is driven by receiving power from a motor shaft of the motor and that reduces rotation of the motor shaft and outputs the reduced rotation to an output shaft; a drive shaft received in the motor housing, connected to the output shaft of the speed reducer via a power transmitter and that drives a driven body; a support shaft that is installed on a bottom wall of the motor housing and that is configured to rotatably support a rotor of the motor; by being inserted into the motor shaft; a cylindrical section to which a stator of the motor is fixed and that is installed on the bottom wall of the motor housing and that is vertically installed to surround the support shaft; a second opening section that is installed on the bottom wall of the motor housing and that is curved in a fan shape so as to surround the cylindrical section; a sensor board that is attached to the second opening section and that comprises a detection element configured to detect a rotation angle of the rotor; and an operation driver that is installed at a place which faces the drive shaft on the opposite side of the drive shaft compared to the sensor board, in the housing cover and that is configured to drive and control the motor, wherein the motor shaft and the output shaft are disposed coaxially, and the output shaft and the drive shaft are disposed parallel to each other and apart in a radial direction. a casing divided into two parts including a box-shaped motor housing having a first opening section formed in one surface thereof, and a housing cover configured to cover the first opening section; a motor received in the motor housing; a speed reducer received in the motor housing and that is driven by receiving power from a motor shaft of the motor and that reduces rotation of the motor shaft and outputs the reduced rotation to an output shaft; a drive shaft received in the motor housing, connected to the output shaft of the speed reducer via a power transmitter and that drives a driven body; a support shaft that is installed on a bottom wall of the motor housing and that is configured to rotatably support a rotor of the motor; by being inserted into the motor shaft; a cylindrical section to which a stator of the motor is fixed and that is installed on the bottom wall of the motor housing and that is vertically installed to surround the support shaft; a second opening section that is installed on the bottom wall of the motor housing and that is curved in a fan shape so as to surround the cylindrical section; a sensor board that is attached to the second opening section and that comprises a detection element configured to detect a rotation angle of the rotor; and an operation driver that is installed at a place which faces the drive shaft on the opposite side of the drive shaft compared to the sensor board, in the housing cover and that is configured to drive and control the motor, wherein the motor shaft and the output shaft are disposed coaxially, and the output shaft and the drive shaft are disposed parallel to each other and apart in a radial direction.