A gear unit is provided with a shaft, a gear, a first shaft support member, a second shaft support member, and a stopper. The gear is supported by the shaft. The gear is capable of rotating and moving along the shaft. The first shaft support member supports one end of the shaft. The second shaft support member supports the other end of the shaft. The stopper is supported by the shaft. The stopper is disposed between the gear and the second shaft support member. The stopper is capable of moving along the shaft. The stopper is configured such that the stopper does not drop off from the shaft in a case where the stopper moves toward the other end of the shaft in a state where the second shaft support member is not supporting the other end of the shaft. The stopper makes contact with the second shaft support member in a case where the stopper moves toward the other end of the shaft in a state where the second shaft support member is supporting the other end of the shaft. The gear makes contact with the stopper in a case where the gear moves toward the other end of the shaft.