An engine unit sensor installation structure comprising: a crankcase including a crank chamber that houses a crankshaft, and an opening through which an end portion of the crankshaft protrudes; a rotary electric machine including a rotor having a cylindrical shape with a bottom, the rotor being attached to the end portion of the crankshaft protruding through the opening, and a stator attached to an non-rotating part close to the rotor; a position detection sensor configured to detect a rotational position of the rotor; an engine cover attached to a wall portion having the opening of the crankcase and covering the end portion of the crankshaft and the outside of the rotary electric machine; a permanent magnet in which the position detection sensor is provided on the rotor; and a Hall IC unit configured to detect the change of magnetic flux in accordance with a rotational position of the permanent magnet on the rotor, wherein: a partition wall that separates the crank chamber from an inner space of the engine cover is provided on the opening of the crankcase; the rotor is attached to the end portion of the crankshaft such that a bottom wall is located on the crankcase side; the stator of the rotary electric machine and the Hall IC unit of the position detection sensor are attached to the engine cover; and an air guiding member for forced cooling used to introduce a cooling air to a space between the partition wall and the bottom wall of the rotor is provided between the partition wall and the rotor, wherein the air guiding member is a rotary vane (67, 77) provided on the bottom wall of the rotor, characterized in that a flow adjusting vane configured to adjust a flow of a cooling air that is sent from the rotary vane (67, 77) is provided in a protruding manner on a surface that faces the bottom wall of the rotor of the partition wall. a crankcase including a crank chamber that houses a crankshaft, and an opening through which an end portion of the crankshaft protrudes; a rotary electric machine including a rotor having a cylindrical shape with a bottom, the rotor being attached to the end portion of the crankshaft protruding through the opening, and a stator attached to an non-rotating part close to the rotor; a position detection sensor configured to detect a rotational position of the rotor; an engine cover attached to a wall portion having the opening of the crankcase and covering the end portion of the crankshaft and the outside of the rotary electric machine; a permanent magnet in which the position detection sensor is provided on the rotor; and a Hall IC unit configured to detect the change of magnetic flux in accordance with a rotational position of the permanent magnet on the rotor, wherein: a partition wall that separates the crank chamber from an inner space of the engine cover is provided on the opening of the crankcase; the rotor is attached to the end portion of the crankshaft such that a bottom wall is located on the crankcase side; the stator of the rotary electric machine and the Hall IC unit of the position detection sensor are attached to the engine cover; and an air guiding member for forced cooling used to introduce a cooling air to a space between the partition wall and the bottom wall of the rotor is provided between the partition wall and the rotor, wherein the air guiding member is a rotary vane (67, 77) provided on the bottom wall of the rotor, characterized in that a flow adjusting vane configured to adjust a flow of a cooling air that is sent from the rotary vane (67, 77) is provided in a protruding manner on a surface that faces the bottom wall of the rotor of the partition wall. a partition wall that separates the crank chamber from an inner space of the engine cover is provided on the opening of the crankcase; the rotor is attached to the end portion of the crankshaft such that a bottom wall is located on the crankcase side; the stator of the rotary electric machine and the Hall IC unit of the position detection sensor are attached to the engine cover; and an air guiding member for forced cooling used to introduce a cooling air to a space between the partition wall and the bottom wall of the rotor is provided between the partition wall and the rotor, wherein the air guiding member is a rotary vane (67, 77) provided on the bottom wall of the rotor, characterized in that a flow adjusting vane configured to adjust a flow of a cooling air that is sent from the rotary vane (67, 77) is provided in a protruding manner on a surface that faces the bottom wall of the rotor of the partition wall.