A synthetic resin container lid having a lid body, and being configured such that the lid body includes a main portion for covering a top face of a mouth-neck section of a container, a cylindrical side wall suspending from the main portion, and a cylindrical surrounding wall surrounding the side wall, locking means extending in a circumferential direction is formed on an inner peripheral surface of the side wall, and the lid body is fitted onto the mouth-neck section of the container to engage the locking means with a locking jaw portion formed on an outer peripheral surface of the mouth-neck section of the container, whereby the lid body is mounted on the mouth-neck section of the container, wherein a surrounding wall separation region, a transition region, and a side wall separation region are disposed sequentially in the counterclockwise direction as viewed from above, in the surrounding wall separation region, an upper end of the surrounding wall is connected to the side wall via a plurality of breakable connecting pieces arranged at intervals in the circumferential direction or via a breakable thin-walled connecting wall extending continuously in the circumferential direction and, at a counterclockwise upstream end of the surrounding wall separation region, a slit extending upwardly from a lower end of the surrounding wall is formed in the surrounding wall, in the transition region and the side wall separation region, an upper end of the side wall is connected to the main portion via a breakable thin-walled connection wall extending continuously in the circumferential direction, and in the transition region, moreover, a breakable thin-walled line is formed in the side wall, and the side wall and the surrounding wall are firmly connected together on a side counterclockwise downstream of the breakable thin-walled line, characterized in that at least a part of the breakable thin-walled line extends obliquely downwardly in the conterclockwise direction, wherein in the transition region, a thin-walled portion, which is defined by locally increasing an inner diameter of the side wall and extends obliquely downwardly in the counterclockwise direction, is formed in the side wall, a region where the side wall and the surrounding wall are firmly connected together includes a part of the thin-walled portion, and the breakable thin-walled line is defined by a part in the thin-walled portion where the side wall and the surrounding wall are not connected together. the lid body includes a main portion for covering a top face of a mouth-neck section of a container, a cylindrical side wall suspending from the main portion, and a cylindrical surrounding wall surrounding the side wall, locking means extending in a circumferential direction is formed on an inner peripheral surface of the side wall, and the lid body is fitted onto the mouth-neck section of the container to engage the locking means with a locking jaw portion formed on an outer peripheral surface of the mouth-neck section of the container, whereby the lid body is mounted on the mouth-neck section of the container, wherein a surrounding wall separation region, a transition region, and a side wall separation region are disposed sequentially in the counterclockwise direction as viewed from above, in the surrounding wall separation region, an upper end of the surrounding wall is connected to the side wall via a plurality of breakable connecting pieces arranged at intervals in the circumferential direction or via a breakable thin-walled connecting wall extending continuously in the circumferential direction and, at a counterclockwise upstream end of the surrounding wall separation region, a slit extending upwardly from a lower end of the surrounding wall is formed in the surrounding wall, in the transition region and the side wall separation region, an upper end of the side wall is connected to the main portion via a breakable thin-walled connection wall extending continuously in the circumferential direction, and in the transition region, moreover, a breakable thin-walled line is formed in the side wall, and the side wall and the surrounding wall are firmly connected together on a side counterclockwise downstream of the breakable thin-walled line, characterized in that at least a part of the breakable thin-walled line extends obliquely downwardly in the conterclockwise direction, wherein in the transition region, a thin-walled portion, which is defined by locally increasing an inner diameter of the side wall and extends obliquely downwardly in the counterclockwise direction, is formed in the side wall, a region where the side wall and the surrounding wall are firmly connected together includes a part of the thin-walled portion, and the breakable thin-walled line is defined by a part in the thin-walled portion where the side wall and the surrounding wall are not connected together.