There is provided an electroless plating bath which makes it possible to form a diffusion barrier layer of a Re-based alloy, having a uniform thickness regardless of the shape and size of a workpiece, on the surface of a Ni-based alloy by a relatively simple method. The electroless plating bath for forming a Ni—Re—B alloy, containing not less than 50 at % of Re, on a substrate by electroless plating, has a pH of 6 to 8 and includes a metal supply source component containing Ni2+ and ReO4− at an equal equivalent in the range of 0.01 to 0.5 mol/L, a complexing agent component containing citric acid and at least one other organic acid, and a reducing agent component containing dimethylamine-borane.