A synthetic resin container lid (2) has a shape including a body (4), which is mounted on a mouth-neck portion (56) of a container by locking a locking means (26) to a locked means (58) of the mouth-neck portion (56), and an upper lid (6) coupled to the body (4) via a hinge means (32). The synthetic resin container lid (2) is improved so that the entire container lid (2) is fully and easily detached from the mouth-neck portion of the container.The axial depth of an annular or arcuate groove (46) formed in a hanging wall (10) of the body (4) and extending in a circumferential direction is changed as required. At least in a second break region (B) and a third break region (C), a circumferential breakable line (53) extending continuously in the circumferential direction along an axially lower end part of the groove (46) is formed in the hanging wall (10).