A buckle includes a base having a plate. A compartment is defined between upper and lower walls of the plate. A front end of a pressing board is engaged with a front end of the plate. A rear end of the pressing board includes a pressing face and two guiding faces. An operating button is received in the compartment and includes a resilient section having upper and lower buttons extending through slots in the upper and lower walls. The operating button can be pressed to cause two pressing surfaces of the resilient section to press against the guiding faces of the pressing board for moving the pressing board rearward relative to the base, disengaging the pressing face of the pressing board from the head strap to permit the head strap to move in a reverse direction for loosening the head strap.
