In a buckle, in which a housing of a female body and an inserting plate of a male body are engaged with each other and which is provided with a simple locking mechanism by using a key, the housing of the female body is provided with an engaging portion, while the inserting plate of the male body is provided with an engaged portion. Alternatively, the housing is provided with an engaged portion while the inserting plate is provided with an engaging portion. Therefore, the female body and the male body can be engaged with each other to be locked. When they are locked, a belt adjusting portion of the male body is concealed in the housing of the female body, so that a belt is difficult to be adjusted. Specifically, the belt can be never loosened but only adjustable in a direction of fastening. Thus, the buckle is provided with a convenient belt adjusting portion, which can not adjust a length of the belt when the female body and the male body are locked with each other.