Swimming/diving goggles includes a main body with a head strap coupling portion. An attachment portion is formed on an end of a head strap. A buckle includes two sidewalls defining a space. An eccentric axle extends between upper front portions of the sidewalls and engages with the attachment portion. Front and rear posts extend between the sidewalls and define a passage. The end of the head strap extends through a rear opening of the buckle into the space, extends through the passage, and is wound around to form a bend. The head strap includes an inner section and an outer section having the attachment portion. The head strap and the buckle are jointly movable. A tightening force acting on the inner section makes the inner section form another bend in the passage. The inner section presses against the front post and the rear post to position the head strap.