A method for manufacturing a joint using a mold, wherein the joint includes: a joint body; and an inner tubular portion and an outer tubular portion formed integrally with the joint body, the inner tubular portion and the outer tubular portion forming an insertion space for receiving a pipe, the inner tubular portion has an outer peripheral surface, a fitting groove is formed in the outer peripheral surface of the inner tubular portion, a sealing member is fitted in the fitting groove to maintain water-tightness in the gap between the pipe received in the insertion space and the inner tubular portion, and a pair of view windows are formed in the outer tubular portion and arranged at opposite positions to visually check the fitting groove and fitting of the sealing member in the fitting groove from the exterior, the view windows being formed such that, when the sealing member is not fitted, the entire circumference and the entire width of the fitting groove can be seen, wherein the mold includes: a pair of split mold members (31a, 31b) for forming an outer peripheral surface of the joint body; an insert passed through the split mold members (31a, 31b) to form the fitting groove in the inner tubular portion; and a slide core (33, 38) for forming the insertion space and a passage inside the inner tubular portion, wherein the insert is configured by a pair of insert members (41a, 41b), projecting portions for forming the view windows are formed on opposing surfaces of the insert members (41a, 41b) each of the projecting portions has a semi-cylindrical distal end, a protrusion for forming the fitting groove is formed in an inner peripheral surface of the distal end of each projecting portion to form the fitting groove in the outer peripheral surface of the inner tubular portion, the method being characterized by: clamping the mold; and injecting molten plastic into a cavity formed in the mold to perform molding.. a joint body; and an inner tubular portion and an outer tubular portion formed integrally with the joint body, the inner tubular portion and the outer tubular portion forming an insertion space for receiving a pipe, the inner tubular portion has an outer peripheral surface, a fitting groove is formed in the outer peripheral surface of the inner tubular portion, a sealing member is fitted in the fitting groove to maintain water-tightness in the gap between the pipe received in the insertion space and the inner tubular portion, and a pair of view windows are formed in the outer tubular portion and arranged at opposite positions to visually check the fitting groove and fitting of the sealing member in the fitting groove from the exterior, the view windows being formed such that, when the sealing member is not fitted, the entire circumference and the entire width of the fitting groove can be seen, wherein the mold includes: a pair of split mold members (31a, 31b) for forming an outer peripheral surface of the joint body; an insert passed through the split mold members (31a, 31b) to form the fitting groove in the inner tubular portion; and a slide core (33, 38) for forming the insertion space and a passage inside the inner tubular portion, wherein a pair of split mold members (31a, 31b) for forming an outer peripheral surface of the joint body; an insert passed through the split mold members (31a, 31b) to form the fitting groove in the inner tubular portion; and a slide core (33, 38) for forming the insertion space and a passage inside the inner tubular portion, wherein the insert is configured by a pair of insert members (41a, 41b), projecting portions for forming the view windows are formed on opposing surfaces of the insert members (41a, 41b) each of the projecting portions has a semi-cylindrical distal end, a protrusion for forming the fitting groove is formed in an inner peripheral surface of the distal end of each projecting portion to form the fitting groove in the outer peripheral surface of the inner tubular portion, the method being characterized by: clamping the mold; and injecting molten plastic into a cavity formed in the mold to perform molding.. clamping the mold; and injecting molten plastic into a cavity formed in the mold to perform molding..