An extension device for dental handpieces for extending the effective length of a dental tool to be clamped within the dental handpiece. The extension device includes an extension unit including a shaft at one end to be clamped within the dental handpiece instead of a dental tool, and a sleeve having a radially-contractible section at the opposite end for receiving a dental tool; and a clamping unit applicable over the radially-contractible section of the sleeve, after having received a dental tool, for securely clamping the dental tool within the sleeve. Also described are a high-speed dental handpiece in which the extension device is particularly useful, and an arrangement for providing lateral support to the extension device when included in the dental handpiece.