A gait motion assisting apparatus (100A, 100B) configured to be attached to a knee-ankle-foot orthosis including a thigh attachment to which a user's thigh can be attached, a thigh frame supporting the thigh attachment and extending in a substantially vertical direction, a lower leg attachment to which the user's lower leg can be attached and a lower leg frame supporting the lower leg attachment and extending in a substantially vertical direction, the lower leg frame being rotatable relative to the thigh frame around a brace-side pivot axis line that is coaxial with a swing axis line of the user's knee joint when the knee-ankle-foot orthosis is attached, the gait motion assisting apparatus comprises an electric motor; a casing housing the electric motor; a driving arm driven and rotated around an actuator-side pivot axis line by rotational power operatively transmitted via a transmission mechanism from an output shaft of the electric motor; an upper connecting mechanism connecting an upper part of the casing to the thigh frame; a lower connecting mechanism operatively connecting a distal end portion of the driving arm to the lower leg frame; and an intermediate connecting mechanism having the actuator-side pivot axis line arranged coaxially with the brace-side pivot axis line, the casing comprises a frame supporting the electric motor and a cover defining an accommodating space for the electric motor and the frame, the electric motor includes a motor body supported by the frame, and the output shaft extends downward from the motor body, the transmission mechanism includes a driving-side bevel gear supported by the output shaft so as to be incapable of relative rotation, and a driven-side bevel gear configured to be closer to the knee-ankle-foot orthosis in a user direction than the output shaft is when the gait motion assisting apparatus is attached to the knee-ankle-foot orthosis and is arranged coaxially with the actuator-side pivot axis line while being engaged with the driving-side bevel gear, the driving arm has a proximal end portion connected to the driven-side bevel gear, the intermediate connecting mechanism includes a ball stud configured to be attached to the knee-ankle-foot orthosis so as to extend outward in the user width direction on the brace-side pivot axis line, and an accommodation depression arranged so as to open toward the knee-ankle-foot orthosis on the actuator-side pivot axis line, the ball stud capable of being inserted into the accommodation depression, and the accommodation depression is arranged in the proximal end portion of the driving arm or at an inner end surface of the driven-side bevel gear. an electric motor; a casing housing the electric motor; a driving arm driven and rotated around an actuator-side pivot axis line by rotational power operatively transmitted via a transmission mechanism from an output shaft of the electric motor; an upper connecting mechanism connecting an upper part of the casing to the thigh frame; a lower connecting mechanism operatively connecting a distal end portion of the driving arm to the lower leg frame; and an intermediate connecting mechanism having the actuator-side pivot axis line arranged coaxially with the brace-side pivot axis line, the casing comprises a frame supporting the electric motor and a cover defining an accommodating space for the electric motor and the frame, the electric motor includes a motor body supported by the frame, and the output shaft extends downward from the motor body, the transmission mechanism includes a driving-side bevel gear supported by the output shaft so as to be incapable of relative rotation, and a driven-side bevel gear configured to be closer to the knee-ankle-foot orthosis in a user direction than the output shaft is when the gait motion assisting apparatus is attached to the knee-ankle-foot orthosis and is arranged coaxially with the actuator-side pivot axis line while being engaged with the driving-side bevel gear, the driving arm has a proximal end portion connected to the driven-side bevel gear, the intermediate connecting mechanism includes a ball stud configured to be attached to the knee-ankle-foot orthosis so as to extend outward in the user width direction on the brace-side pivot axis line, and an accommodation depression arranged so as to open toward the knee-ankle-foot orthosis on the actuator-side pivot axis line, the ball stud capable of being inserted into the accommodation depression, and the accommodation depression is arranged in the proximal end portion of the driving arm or at an inner end surface of the driven-side bevel gear.