Abstract: Conventionally, to lock a cap onto a container with a minute interval of degree rotation, it is necessary to provide a large number of engaging teeth at minute intervals and to engage minute locking teeth thereto, but there is the problem that adequate locking force is not attained so the lock is disconnected, making a secure lock impossible. There is also the problem that high-precision forming technology is required, making it difficult to provide the neck or shoulders of a container with teeth using a plastic blow-forming process. A cap locking device in which engaging teeth provided in a circumferential shape on one side are elastically engaged by locking teeth 2 provided on the other side when a cap 10 is rotatingly mounted onto a container 20, wherein locking teeth B1 and C1 positioned on both sides of a central locking tooth A1 of a set of three locking teeth 2 are set to an integer multiple pitch interval including zero of engaging teeth plus one-third or two-thirds of a pitch interval.

Abstract: If the diameter of an opening side neck portion is decreased to minimize the contact surface between contents and air and a small cap is fitted on a container where there is the concern that exposure to air may alter the quality of contents in the container, there is not only the problem that a gap results between the cap top board and the container opening where adequate locking force is not available but the problem that force is required to open and close the lid, making removal and fitting troublesome. A container made so a large cap can be fitted thereto by providing a cap locking neck portion 30 whose inner and outer diameters are larger than an opening side neck portion 13 at a container should side neck portion 14 of a container neck 10.