A drive unit provided for an electric power-assisted bicycle that can travel with a combination of a human driving force generated by a pedal force from a pedal and an auxiliary driving force generated by a motor, the drive unit comprising a crank shaft rotatable by a human driving force, a resultant-force transmission member rotatable by a resultant driving force of a human driving force and an auxiliary driving force, an output wheel member provided on the resultant-force transmission member, and a speed change mechanism that changes a rotation speed of the resultant-force transmission member, the crank shaft and the resultant-force transmission member being coaxial with each other, and the speed change mechanism having a plurality of selectable gears with different gear ratios, characterized in the drive unit further comprising a detection unit that detects selected one of the gears, wherein a gear shifting state between the gears before and after a gear shift is detectable by the detection unit at a gear shift, and when the gear changing state is detected by the detection unit, the motor is controlled so as to temporarily reduce an auxiliary driving force, wherein the gears include selectable low and high gears, the speed change mechanism includes a low-speed gear, a high-speed gear, and a gear shift mechanism that shifts the gears, the low-speed gear and the high-speed gear interlock with the motor, the gear shift mechanism includes a movable shifting member that is movable to a low-gear shift position and a high-gear shift position, if the low gear is selected, the movable shifting member moves to the low-gear shift position and a resultant driving force is transmitted to the resultant-force transmission member through the low-speed gear so as to rotate the resultant-force transmission member, and if the high gear is selected, the movable shifting member moves to the high-gear shift position and a resultant driving force is transmitted to the resultant-force transmission member through the high-speed gear so as to rotate the resultant-force transmission member, and wherein the detection unit detects a shifting position of the movable shifting member so as to identify the selected gear, and the detection unit detects an intermediate position between the low-gear shift position and the high-gear shift position of the movable shifting member so as to recognize a gear shifting state. the drive unit comprising a crank shaft rotatable by a human driving force, a resultant-force transmission member rotatable by a resultant driving force of a human driving force and an auxiliary driving force, an output wheel member provided on the resultant-force transmission member, and a speed change mechanism that changes a rotation speed of the resultant-force transmission member, the crank shaft and the resultant-force transmission member being coaxial with each other, and the speed change mechanism having a plurality of selectable gears with different gear ratios, the drive unit further comprising a detection unit that detects selected one of the gears, wherein a gear shifting state between the gears before and after a gear shift is detectable by the detection unit at a gear shift, and when the gear changing state is detected by the detection unit, the motor is controlled so as to temporarily reduce an auxiliary driving force, wherein the gears include selectable low and high gears, the speed change mechanism includes a low-speed gear, a high-speed gear, and a gear shift mechanism that shifts the gears, the low-speed gear and the high-speed gear interlock with the motor, the gear shift mechanism includes a movable shifting member that is movable to a low-gear shift position and a high-gear shift position, if the low gear is selected, the movable shifting member moves to the low-gear shift position and a resultant driving force is transmitted to the resultant-force transmission member through the low-speed gear so as to rotate the resultant-force transmission member, and if the high gear is selected, the movable shifting member moves to the high-gear shift position and a resultant driving force is transmitted to the resultant-force transmission member through the high-speed gear so as to rotate the resultant-force transmission member, and wherein the detection unit detects a shifting position of the movable shifting member so as to identify the selected gear, and the detection unit detects an intermediate position between the low-gear shift position and the high-gear shift position of the movable shifting member so as to recognize a gear shifting state.