To provide a built-in spacer of a reinforcement binding unit capable of being used for a binding device of reinforcements in order to use the built-in spacer mounted in a dot-like shape around the reinforcement binding unit incorporated into a form.

The spacer comprises a combination of a small block-shaped spacer body 1 slightly longer than the width of a gap formed between the reinforcement on the circumference of the reinforcement binding unit and the internal wall surface of the form with a hook member 2 projected to the base end face side of the spacer body and for firmly hooking the spacer body to the reinforcement, the hooking member 2 formed by bending a metal wire rod is so constituted that both leg ends of the body formed in a U-shape as a whole is folded back oppositely and slantly inward to provide a reinforcement holding claw 2b, and a connecting bar 2c for connecting base ends of both legs is partially embedded into the spacer body. The height of the hooking member 2 becomes a size enabling a cross-shaped cross position of the reinforcements to be held between the front end of the reinforcement holding claw 2b and the base end face 1a of the spacer body 1.