A robot hand for assembling a transmission gear is provided to improve productivity in a transmission assembling line by allowing a differential gear to be easily engaged with a transfer gear. A robot hand(100) for assembling a transmission gear includes a rotation support unit(110) and a spacer unit(120). The rotation support unit connects a robot to the robot hand and supports the robot hand in such a manner that the robot hand rotates about a central line extending through the center of a differential assembly fixed by a finger(101). The spacer unit allows the robot hand to rotate about the rotation support unit within a predetermined range in a state in which a differential assembly(20) is elastically supported by the spacer unit.