Abstract: A dielectric barrier discharge device in an embodiment includes: a dielectric having a hollow-shaped flow path; a first electrode and a second electrode provided apart along the dielectric so as to cause a first region in which plasma is formed inside the flow path; and a power supply to apply a voltage between the first electrode and the second electrode. The dielectric includes a flow path area adjusting portion provided to project from an inner wall of the dielectric toward a center of the flow path in a manner that a first flow path cross-sectional area in the first region is smaller than a second flow path cross-sectional area in a second region other than the first region in the flow path.