To provide a plunger surface roughness adjusting device for allowing easy replacement of a cutting member.

The plunger surface roughness adjusting device comprises a rotating member 30 arranged around the outer peripheral face of a plunger 3 and preventive of movement in the axial direction of the plunger 3, the cutting member 50 provided inside the rotating member 30 for abutting on the plunger 3, and a driving means for rotationally driving the rotating member 30 around the plunger 3. As the driving means rotationally drives the rotating member 30, the cutting member 50 cuts the outer peripheral face of the plunger 3 to increase the surface roughness of the outer peripheral face. The cutting member 50 can be mounted/demounted on/from the rotating member 30 with a hook-and-loop fastener 53.