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.