A refrigerator comprising: a cabinet that defines a storage chamber; a drawer door configured to be inserted into and withdrawn out of the storage chamber, the drawer door comprising a drawer part that defines an upwardly open storage space, and a door part that is configured to, based on the drawer door being inserted into the storage chamber, close the storage chamber; a motor assembly provided at the storage chamber and including a motor and a pinion gear that is rotated by the motor, the motor assembly being configured to provide a driving force that moves the drawer door into and out of the storage chamber; and a rack gear assembly provided at the drawer door and having an extendable rack that is configured to be coupled to the pinion gear, the refrigerator further comprises: a guide unit that is provided at an inner surface of the storage chamber and configured, based on the drawer door being withdrawn out of the storage chamber, to couple a portion of the rack gear assembly to the inner surface of the storage chamber to thereby allow the extendable rack to extend, wherein the guide unit comprises: a guide fixing portion which is mounted on the bottom surface of the storage chamber and in which a guide body through-hole that is vertically penetrated is defined; a guide body which is elevatably provided on the guide fixing portion and of which a portion passes through the guide body through-hole to protrude to the inside of the storage chamber so as to be inserted into a receiving portion; and an elastic portion configured to elastically support the guide body, the elastic portion being compressed when the guide body descends; wherein the rack gear assembly further comprises: a fixed rack portion that is fixed to the drawer door, a moving rack portion configured to extend rearward from the fixed rack portion, and a fixing member provided at the moving rack portion and defining the receiving portion that is configured to receive the guide unit, and the guide unit is provided to be elastically movable toward the receiving portion so as to be inserted into the receiving portion in a state in which the drawer door is completely inserted into the storage chamber to allow the rack gear assembly to be minimized in length. a cabinet that defines a storage chamber; a drawer door configured to be inserted into and withdrawn out of the storage chamber, the drawer door comprising a drawer part that defines an upwardly open storage space, and a door part that is configured to, based on the drawer door being inserted into the storage chamber, close the storage chamber; a motor assembly provided at the storage chamber and including a motor and a pinion gear that is rotated by the motor, the motor assembly being configured to provide a driving force that moves the drawer door into and out of the storage chamber; and a rack gear assembly provided at the drawer door and having an extendable rack that is configured to be coupled to the pinion gear, the refrigerator further comprises: a guide unit that is provided at an inner surface of the storage chamber and configured, based on the drawer door being withdrawn out of the storage chamber, to couple a portion of the rack gear assembly to the inner surface of the storage chamber to thereby allow the extendable rack to extend, wherein the guide unit comprises: a guide fixing portion which is mounted on the bottom surface of the storage chamber and in which a guide body through-hole that is vertically penetrated is defined; a guide body which is elevatably provided on the guide fixing portion and of which a portion passes through the guide body through-hole to protrude to the inside of the storage chamber so as to be inserted into a receiving portion; and an elastic portion configured to elastically support the guide body, the elastic portion being compressed when the guide body descends; a guide fixing portion which is mounted on the bottom surface of the storage chamber and in which a guide body through-hole that is vertically penetrated is defined; a guide body which is elevatably provided on the guide fixing portion and of which a portion passes through the guide body through-hole to protrude to the inside of the storage chamber so as to be inserted into a receiving portion; and an elastic portion configured to elastically support the guide body, the elastic portion being compressed when the guide body descends; wherein the rack gear assembly further comprises: a fixed rack portion that is fixed to the drawer door, a moving rack portion configured to extend rearward from the fixed rack portion, and a fixing member provided at the moving rack portion and defining the receiving portion that is configured to receive the guide unit, and the guide unit is provided to be elastically movable toward the receiving portion so as to be inserted into the receiving portion in a state in which the drawer door is completely inserted into the storage chamber to allow the rack gear assembly to be minimized in length. a fixed rack portion that is fixed to the drawer door, a moving rack portion configured to extend rearward from the fixed rack portion, and a fixing member provided at the moving rack portion and defining the receiving portion that is configured to receive the guide unit, and the guide unit is provided to be elastically movable toward the receiving portion so as to be inserted into the receiving portion in a state in which the drawer door is completely inserted into the storage chamber to allow the rack gear assembly to be minimized in length.