A strap adjusting device includes a buckle having two through-holes spaced by a partitioning wall. A pressing member is slideably and rotatably received in each through-hole. The pressing members are movable relative to the buckle to adjust a distance between each pressing member and a pressing edge in each through-hole. At least one resilient strap is mounted to a body of a pair of swimming goggles and includes two ends each of which is wound around one of the pressing members. The strap is pullable to adjust tightness of the strap when the pair of swimming goggles is worn on the head of the user. The pulling force acting on the strap urges each pressing member to move toward one of the pressing edges to tightly clamp the strap and to fix a length of the strap.