PROBLEM TO BE SOLVED: To provide a construction method for an isolation layer easily and surely attaining isolation using a super-retarder. SOLUTION: Unwoven fabric 6 is stuck as a permeation material to the surface of shotcrete 1 sprayed onto the excavated natural ground. An isolation sheet 1 comprising a waterproof sheet 2 and a sheet 4 filled with the super- retarder 3 is stuck onto the unwoven fabric 6 so that the waterproof sheet 2 is on the unwoven fabric 6 side, and lining concrete 7 is further placed on the isolation sheet 1. Before the lining concrete 7 is hardened, the sheet 4 with the super-retarder 3 is dissolved to allow the super-retarder 3 to flow out, thus forming a hardening failure part 8 in the lining concrete 7. The hardening failure part 8 isolates the shotcrete 5 from the lining concrete 7.