A concentrating apparatus for compressed air comprising: a concentrating tube into which compressed air can be introduced directly and which is formed by coaxially connecting a plurality of concentrating units with each other, the plurality of concentrating units which are arranged on a compressed air movement passage, each unit being provided therein with a collision wall capable of allowing compressed air to collide therewith, which is arranged at an upstream side within the concentrating unit and which is formed in a disc shape similar to a cross section of the concentrating unit, wherein a collision chamber is defined by the collision wall and an upstream side inner surface of the concentrating unit, wherein the concentrating unit is arranged at a most downstream side end part thereof with an upstream side end face of an adjacent downstream side concentrating unit, wherein the concentrating unit is provided at an upstream side end face thereof with an inlet tube projecting therefrom, and further provided at a downstream side inner part thereof with an outlet tube, an upstream side end part of the outlet tube is arranged near the other side surface of the collision wall, and the upstream side end face of the outlet tube and the downstream side end part of the concentrating unit are partitioned with a conical partition wall, and an annular narrow passage which is formed between an outer periphery of the collision wall and the inner surface of the side part of the concentrating unit and which is capable of jetting compressed air, the compressed air being collided with the collision wall so that the compressed air is concentrated and can be partially liquefied, the compressed air being jetted from the annular narrow passage so as to be adiabatically expanded and partially liquefied, wherein the concentrating unit is further provided therein with a bent passage configured to push the compressed air therein, so that the compressed air is pressurized, and the compressed air, which has been pressurized in the bent passage, is pushed back toward the collision wall side so as to be collided with the other side of the collision wall, so that the compressed air is concentrated and can be partially liquefied, wherein the bent passage has an acute cross section being formed in a V shape of the inner surface of an end part of the side peripheral surface of the concentrating unit and the inner surface of the partition wall, wherein: a) each concentrating unit of a horizontally installed concentrating tube is attached at a bottom part thereof with a drain trap, so that drain can be collected; or b) the concentrating tube is horizontally installed and an engagement wall at a lower part of each concentrating unit and an end face of the adjacent downstream side concentrating unit, which is engaged with the engagement wall are formed with water passage holes (86, 87) so as to be communicated with each other such that the drain, which has generated in each concentrating unit is flowed down to an inner surface at a lower part of the adjacent concentrating unit, and collected and discharged by a drain trap together with the drain 26 remained in the most downstream side concentrating unit; or c) the concentrating tube is vertically installed and the concentrating unit in the lowest position is provided with a drain trap. a) each concentrating unit of a horizontally installed concentrating tube is attached at a bottom part thereof with a drain trap, so that drain can be collected; or b) the concentrating tube is horizontally installed and an engagement wall at a lower part of each concentrating unit and an end face of the adjacent downstream side concentrating unit, which is engaged with the engagement wall are formed with water passage holes (86, 87) so as to be communicated with each other such that the drain, which has generated in each concentrating unit is flowed down to an inner surface at a lower part of the adjacent concentrating unit, and collected and discharged by a drain trap together with the drain 26 remained in the most downstream side concentrating unit; or c) the concentrating tube is vertically installed and the concentrating unit in the lowest position is provided with a drain trap.