oil well or a natural gas well with drilling mud 1 containing bentonite or the like introduced into the hole 2 as in the case shown in fig1 a referring to fig3 b when the bottom of the hole 2 encounters a drilling mud leaking position a the drill bit 3a is slightly withdrawn upward and a filler carrying a chemical preparation including a curing reaction promoting catalyst is introduced as mixed into drilling mud 1 39 to the interior of the drill rod 3b and is allowed to reach the depth of the hole 2 consequently the filler supplied to the hole 2 flows together with the drilling mud 1 to the leaking position a and entwines on a location relatively close to the opening of the leaking position a to form a layer of filter there as seen from fig4 a referring to fig3 c a spacer liquid 4 39 comprising liquid paraffin or the like which floats on the drilling mud and does not react with the chemical liquid as in the case shown in fig1 b is next introduced into the drill rod 3b and thereafter the chemical liquid 5 39 is introduced into the drill rod 3b the drilling mud is driven out of the drill rod 3b by the spacer liquid 4 39 in order that the chemical liquid 5 39 does not start the curing reaction inside the drill rod 3b thus the chemical liquid 5 39 is fed via the drill bit 3a into.