Watertight fastener and method for manufacturing same

A watertight fastener includes a pair of fastener tapes; fastener elements provided on a side of one end edge part along a longitudinal direction of the fastener tapes; and a slider engaging and disengaging the fastener elements. On a surface of the fastener tapes opposite to the fastener elements, a waterproof seal layer is provided along the longitudinal direction at a position at which the fastener elements are provided. A width of the waterproof seal layer in a direction orthogonal to the longitudinal direction is narrower than a width of the slider. With the pair of fastener tapes being closed by engagement of the fastener elements, end parts of the waterproof seal layer close to the end edge parts of the fastener tapes come into contact with each other without overlapping in a thickness direction and without a gap to be in close contact in a liquid-tight manner.