End portions of a sheet-shaped rubber member are manually and firmly joined together. A pair of opening/closing members is arranged to sandwich and move along the end portions. An opening/closing mechanism opens/closes the pair of opening/closing members between an open position and a closed position where the end portions are joined together. A pair of guiding rollers is provided to one opening/closing member and guides each of the end portions to a joining position. A pressing roller is provided to the other opening/closing member and presses each of the end portions onto the guiding roller. A pair of joining rollers is provided to the pair of opening/closing members, respectively, and sandwiches each of the end portions guided by the pair of guiding rollers. The joining rollers roll on each of the opposing end portions and abut and join the end portions to each other.