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. <EOS>