A new concept boiler where fuel can be efficiently combusted by high temperature preheated air is proposed and experimental demonstration is done. This boiler is suitable for low BTU gas derived from gasification process of coal and wastes with no dioxin emission. This boiler is characterized by the following features; uniform heat flux field, augmentation of heat transfer, reduction of combustion noise level, suppression of NOx emission and compactness. Preliminary experiments using natural gas as a fuel demonstrate above distinct performance as well as good agreement with three-dimensional numerical simulation.