To provide a new decompression agent injecting method excellent in permeation effect with low cost comprising the steps of decompressing the inside of timber by a new means and injecting a liquid agent by a pressure difference generated, and provide an agent-impregnated timber.

The decompression agent injecting method comprises the steps of boring the timber to make a through-hole by laser-incising or the like, making steam flow through the hole to make steam stay in the timber or drying the timber to make high temperature steam stay in the timber, and immersing the timber in the liquid agent such as a wood preservative agent or the like, and impregnating the timber with the liquid agent using pressure difference generated by condensation of the steam caused by temperature decrease. The agent-impregnated timber is manufactured by the method.