To provide a method capable of easily forming a water tunnel for connecting water reservoirs by excavating a tunnel in a state to cover a part as a tunnel opening in advance without emptying the water reservoirs.

A freezing means 4 is provided around a tunnel opening planned section 3 of the reservoirs 1 and 2, an area having the freezing means 4 including the tunnel opening planned section 3 is covered with a pouring water pipe 6 attached heat insulating sheet 7, water is poured in the area covered with the heat insulating sheet 7, the water poured in the area is frozen by the freezing means 4 to form ice 8 for covering the tunnel opening planned section 3, the tunnel 10 is bored from the ground, the tunnel opening planned section 3 is opened in a state covered with the ice 8, and after the tunnel 10 has been made to pass through between both water reservoirs 1 and 2, the ice 8 is removed.