An atomizer (10, 10a-10d, 50), comprising: a mixing chamber adapted to form a fluid mixture by mixing a spray fluid as fuel and a spray medium as gas for atomizing the spray fluid; a pair of spray fluid flow passages (23, 23a, 23b, 24, 24a, 24b) adapted to supply the spray fluid to the mixing chamber; a spray medium flow passage (29, 30) adapted to supply the spray medium to the mixing chamber; and one or more outlet holes (41, 42) adapted to spray the fluid mixture to the outside; wherein the mixing chamber includes at least one middle ejection hole adapted to spray the spray fluid to the mixing chamber, the pair of the spray fluid flow passages (23, 23a, 23b, 24, 24a, 24b) is connected to each other on a downstream side of a flow of the spray fluid, the middle ejection hole is arranged at a joining section where the pair of the spray fluid flow passages (23, 23a, 23b, 24, 24a, 24b) is connected to each other, characterized in that when in use, the spray fluid that flows through the pair of the spray fluid flow passages (23, 23a, 23b, 24, 24a, 24b) collides on each other at the joining section of the spray fluid flow passages (23, 23a, 23b, 24, 24a, 24b), and is sprayed from the middle ejection hole so as to form a spray of fan shape, and the spray medium is supplied to the mixing chamber in an axial direction of the atomizer and flows along a side partition wall of the mixing chamber extending along the axial direction of the atomizer, and is mixed with the spray of the spray fluid after attenuation of the flow speed of the spray of the spray fluid, so as to apply a shear force to the surface of the spray fluid due to the flow speed difference between the spray fluid and the spray medium. a mixing chamber adapted to form a fluid mixture by mixing a spray fluid as fuel and a spray medium as gas for atomizing the spray fluid; a pair of spray fluid flow passages (23, 23a, 23b, 24, 24a, 24b) adapted to supply the spray fluid to the mixing chamber; a spray medium flow passage (29, 30) adapted to supply the spray medium to the mixing chamber; and one or more outlet holes (41, 42) adapted to spray the fluid mixture to the outside; wherein the mixing chamber includes at least one middle ejection hole adapted to spray the spray fluid to the mixing chamber, the pair of the spray fluid flow passages (23, 23a, 23b, 24, 24a, 24b) is connected to each other on a downstream side of a flow of the spray fluid, the middle ejection hole is arranged at a joining section where the pair of the spray fluid flow passages (23, 23a, 23b, 24, 24a, 24b) is connected to each other, characterized in that when in use, the spray fluid that flows through the pair of the spray fluid flow passages (23, 23a, 23b, 24, 24a, 24b) collides on each other at the joining section of the spray fluid flow passages (23, 23a, 23b, 24, 24a, 24b), and is sprayed from the middle ejection hole so as to form a spray of fan shape, and the spray medium is supplied to the mixing chamber in an axial direction of the atomizer and flows along a side partition wall of the mixing chamber extending along the axial direction of the atomizer, and is mixed with the spray of the spray fluid after attenuation of the flow speed of the spray of the spray fluid, so as to apply a shear force to the surface of the spray fluid due to the flow speed difference between the spray fluid and the spray medium.