Provided is a bag-making machine for manufacturing a plastic bag with a body part and side gusset parts, in which when a bag having a long height is manufactured, it is not required to use a body member with a long width. Side gusset members (14) are folded in two and fed by a side gusset member supply mechanism (16). The side gusset members (14) are disposed in the longitudinal direction of a body member (11) so as to face each other at spaces therebetween in the lateral direction of the body member (11). Each of the side gusset members (14) is divided into predetermined lengths, and the divided side gusset members (14) are disposed at spaces therebetween in the longitudinal direction of the body member (11). The body member (11) and the side gusset members (14) are so heat-sealed by a vertical sealing device (26) in the longitudinal direction of the body member (11) as to form vertical seal parts (27).