A drift-tube linear accelerator that passes an injected particle beam through inside a plurality of cylindrical drift-tube electrodes arranged in a cylindrical cavity in a particle beam traveling direction and accelerates the particle beam by a radio-frequency electric field generated between the plurality of cylindrical drift-tube electrodes, wherein at least part of a focusing device for focusing the particle beam is disposed inside an end drift-tube electrode that is arranged nearest the injection side of the cylindrical cavity among the plurality of cylindrical drift-tube electrodes, with the focusing device being positionally adjustable independently of the end drift-tube electrode.