Abstract: This add-on automatic clutch device is to be added onto a vehicle having manual transmission and a clutch, and includes: a first hydraulic pipe having one end connected to a clutch master cylinder unit of the vehicle; a second hydraulic pipe having one end connected to a clutch release cylinder unit of the vehicle; a cylinder having a first port connected to the other end of the first hydraulic pipe and a second port connected to the other end of the second hydraulic pipe; a piston provided in the cylinder so as to be able to move back and forth; and a motor that generates drive power to cause the piston to move back and forth. A first port and a second port are disposed such that when the piston reaches the bottom dead center, oil can flow between the first hydraulic pipe and the second hydraulic pipe by the first port and the second port being connected through the cylinder.