To improve safety by a method wherein an inclined shaft tunnel and a horizontal tunnel are excavated from a land and a cover lock is arranged at the forward end of the horizontal tunnel the position of which is specified.

After an inclined shaft tunnel 12 is excavated from a land 10 to the depth of a methane hydrate stratum, a horizontal tunnel 14 is excavated in the direction of the methane hydrate stratum and 70-100 m this side of the methane hydrate stratum. Thereafter, a ground at the forward end of the tunnel 14 is bored and cement is placed, a cover rock 16 is formed to reinforce the ground and effect cut-off. Boring is then effected from the forward end of the tunnel 14 toward the methane hydrate stratum and pipes 20, 22, and 24 are inserted. After high temperature steam is injected in the methane hydrate stratum through the pipe 20 and separated into gas and water, the pipes 22 and 24 are inserted to bleed gas and water and bled gas is transferred through a pipe installed through the tunnels 14 and 12 to the land 10. This constitution performs safe development drilling without heeding an offshore production facility.