A rotational positioning device, comprising: a housing; a rotational member rotatable about a rotational member axial line, at least a part of the rotational member being accommodated in the housing; and at least one sensor installed in the housing, wherein in order to rotate the rotational member, a motor is connected to the rotational member, or the rotational member has a transmission mechanism installed along a circumferential direction of the rotational member, driving force being transmitted to the rotational member via the transmission mechanism; and wherein a graduated scale having a plurality of scale marks is integrally formed with the rotational member along a circumferential direction of the rotational member, the at least one sensor detects an angle change amount due to rotation of the rotational member based on the plurality of scale marks, and the rotational member is to be positioned at a target angle position based on the angle change amount, characterized in that the rotational positioning device further comprises a sensor flange installed in the housing, a circular rotational member hole being provided in the sensor flange, the at least one sensor installed on the sensor flange; wherein at least a portion of the rotational member is housed within the rotational member hole, whereby a central axis line of the rotational member hole is aligned with the rotational member axial line such that a position of the at least one sensor to the graduated scale is determined; and wherein a first seal is provided between the sensor flange and the rotational member to prevent a lubricant from entering a space where the at least one sensor is installed. a housing; a rotational member rotatable about a rotational member axial line, at least a part of the rotational member being accommodated in the housing; and at least one sensor installed in the housing, wherein in order to rotate the rotational member, a motor is connected to the rotational member, or the rotational member has a transmission mechanism installed along a circumferential direction of the rotational member, driving force being transmitted to the rotational member via the transmission mechanism; and wherein a graduated scale having a plurality of scale marks is integrally formed with the rotational member along a circumferential direction of the rotational member, the at least one sensor detects an angle change amount due to rotation of the rotational member based on the plurality of scale marks, and the rotational member is to be positioned at a target angle position based on the angle change amount, characterized in that the rotational positioning device further comprises a sensor flange installed in the housing, a circular rotational member hole being provided in the sensor flange, the at least one sensor installed on the sensor flange; wherein at least a portion of the rotational member is housed within the rotational member hole, whereby a central axis line of the rotational member hole is aligned with the rotational member axial line such that a position of the at least one sensor to the graduated scale is determined; and wherein a first seal is provided between the sensor flange and the rotational member to prevent a lubricant from entering a space where the at least one sensor is installed.