To perform normal and efficient drilling operation without raising problems such as glazing and burn-out of a cutting edge, lingering drilling work or abnormal abrasion in a drilling method of concrete structure using a hand held drill fitted with a non-core bit.

A preliminary hole 6 is formed by a drilling work to a concrete structure 5 using a hand held drill 2 as in shown Fig 2, wherein the hand held drill 2 fitted with a non-core bit 1 having relatively small diameter of about 10 mm in an outer diameter. After that, a non-core bit 7 of about 20 mm outer diameter is fitted in place of the previous non-core bit. The bit 7 is formed of a protrusion 9 set protruded on its tip of cutting edge 8, wherein the protrusion 9 has the same diameter as the previous non-core bit. A hole of enlarged diameter is formed after positioning the protrusion 9 by fitting to the preliminary hole 6, there the preliminary hole 6 for fitting the protrusion 9 guides the drilling.