A rear wall is detachably fitted at the rear of a body frame. A grinding wheel casing is also associated at the rear of the rear wall. A feeder wheel disposed on the front surface of the body frame and a rotatable grinding wheel provided within the grinding wheel casing are respectively rotated by a motor provided within the body frame. A cutter disposed opposite the feeder wheel cuts open a lid of a can rotated by the feeder wheel. The rotatable grinding wheel provided within the grinding wheel casing sharpens the edge of a knife inserted into the grinding wheel casing from a slit formed in a portion of the grinding wheel casing.