The invention relates to an adjuster device (1) for adjusting the length and fastening of a web type strap (12), and in particular to an adjuster device (1) intended to be used together with a child restraint harness as part of a child vehicle seat. The adjuster device according to the invention allows, when the adjuster is in the open, release position, the strap to be easily pulled and freely move through the adjuster in both directions whereas the strap, when the adjuster is in the closed, fastening position, is securely fastened (without any risk for slippage) while still allowing the strap to be easily tightened by pulling the strap in one direction only. The present device does not cause wear and tear of the web, prevents slippage and is easy to use.