A plunger is fittable within a cylinder of a pneumatic dispenser that discharges a viscous material. The plunger has a first portion located at the front, and a second portion located at the rear. The second portion is a hollow structure and has a circumferential wall. An inner circumferential surface of this circumferential wall has a tapered surface. The circumferential wall has a thickness dimension that decreases in the axial direction moving away from the first portion. Therefore, the circumferential wall easily displaces in the radial direction, because the bending stiffness decreases in the axial direction moving away from the first portion. The first portion is a solid structure that is more rigid than the second portion. The first portion also has a partition wall surface that separates the inner chamber of the second portion from the solid section of the first portion.