To provide a new type adjustable reamer which can finely adjust the outside diameter of a cutting surface and further can cope with prepared holes having an inside diameter from a small-to-large diameters.

In the adjustable reamer composed of a hollow cylindrical member which is provided with a threaded hole at a rear end part thereof, a taper hole at a middle part thereof, and a through hole at a tip end part thereof, and serves as a cutting surface having abrasive grains suck on an outside peripheral surface of a middle part cylindrical wall, an adjusting screw rod which is movably screwed into the threaded hole from the rear end side, and a taper pin which is slidably inserted into the taper hole, the hollow cylindrical member is provided with a plurality of longitudinal slits on the middle cylindrical wall in parallel with each other and the outside diameter of the cutting surface can be finely adjusted by sliding the taper pin by turning the adjusting screw rod.