Effect of Cr and Mo contents on the corrosion properties of Ni-Cr-Mo alloys was investigated in the supercritical water including extraction from the fly ash of incinerated municipal solid waste at 873 K/25 MPa. Among the alloys of which Cr content was 25 mass% or less, the more Mo content, the greater the mass gain became. On the contrary, the alloys of which Cr was 30 mass% showed a smaller mass gain as those of the Mo content increased. The alloys of which Cr content was 35 mass% or greater showed their mass gains 1 mg/cm 2 or less for 168-hour exposure. It was concluded that the Ni-Cr-Mo alloys, in order to exhibit preferable corrosion resistance, should contain Cr at least 30 mass % under this environment.