The invention relates to an arrangement for preparing a gas in a closable reactor by supplying the reactor with carbon-based biomass or chopped wood material, such as chips, in substantially oxygen-free conditions, by allowing the biomass or wood material to gasify at a high temperature, and by recovering the gas generated in a gasification reaction. In that the arrangement the reactor has its interior defined by a feed pipe whose inlet end is closable with a shut-off valve, especially with a ball valve, and whose outlet end adjoins a heatable gasification dome, biomass or chopped wood material is delivered from the feed pipe's inlet end into the reactor's interior, the reactor's interior is supplied with free water/water vapor in its supercritical state, which is optionally prepared catalytically by splitting water/water vapor, the biomass or wood material is conveyed into a gasification space of the reactor's interior, which is in connection with the heated gasification dome and which is adapted to have existing conditions selected in a manner such that the water present in said gasification space is present in its supercritical state, and the gas generated in the gasification reaction is recovered.