An assembly system includes a molding machine that molds first parts and second parts and a robot having an assembly unit including a first holder that holds the first parts and a second holder that holds the second parts with each other. The first parts and the second parts that are molded by the molding machine are removed from the mold while being held by the first holder and the second holder of the robot. At least one of the first holder and the second holder is moved relative to the other so as to assemble the first parts and the second parts without releasing the first parts held by the first holder and the second parts held by the second holder.