A drive motor is proposed, which serves as a drive source for a movement mechanism constituting an iris for an optical path of an imaging system. The drive motor has a shaft serving as a center of rotation, a magnet, two-pole magnetized and rotated around the shaft, a turning arm engaged with movable members and turned integrally with the magnet, for moving the movable members in a direction associated with a turning direction, a stator coil wound in a direction perpendicular to a direction of rotation of the magnet and disposed in such a way to enclose the magnet from outside, a coil bobbin on which the stator coil is wound, a stator yoke disposed outside the coil bobbin and having a cylindrical circumferential surface part penetrated in an axial direction of the shaft, and position detection means for detecting a rotational position of the magnet.