To provide a compact size-extension/contraction adjustable buckle having a simple structure.

This buckle 1 of a belt installs the belt by turnably disposing one end of a plate-like frame 2 at one end of a belt 20, disposing a pin 7 at the back side of the other end of the frame, wrapping the belt around the waist part of a wearer to guide the other end to the back side of the plate-like frame, and selectively inserting the pin into one of a plurality of pin insertion holes 21 opened at the other end of the belt. The pin is disposed slidably in the longitudinal direction of the belt relative to the plate-like frame, a spring 8 is disposed between the pin and the plate-like frame and the pin is urged in the fastening direction of the belt by the spring.