A buckle includes a base that receives therein a pressing board having a front end pivotally coupled to the base and a rear end provided with a clutch portion for clutching a tooth of a head strap, and an operating member that is received in the base and corresponds to a back of the pressing board. The operating member has two buttons. When an arcuate bar pressing against the operating member is pressed inward, the arcuate bar deforms and pushes the pressing board to swing outward, making the clutch portion disengage from the tooth of the head strap, so that length adjustment of the head strap is allowed. After the adjustment, the pressing board automatically swings back due to the tension of the head strap, so that the clutch portion clutches another tooth of the head strap to position the head strap.