Fitting tool and bag body with fitting tool

Provided are: an interlocking structure which has superior sealing properties and can sufficiently suppress leakage of a liquid even when the liquid is stored inside a bag body; and a bag body with an interlocking structure that uses this interlocking structure. This interlocking structure (10) comprises: a first interlocking member (12) provided with a first interlocking part (18) on a band-shaped first base material (16); and a second interlocking member (14) provided with a second interlocking part (28) that is freely attachable/detachable to/from the first interlocking part (18) on a band-shaped second base material (26). In a state where the first interlocking part (18) and the second interlocking part (28) are fitted to each other: a third arm part (30) and a fourth arm part (32) are fitted between a first arm part (20) and a second arm part (22); a protrusion part (24) is fitted between the third arm part (30) and the fourth arm part (32); both sides of a head section (24b) are hooked onto projection s