To provide a method for stabilizing unstable α-thrombin in a thrombin-containing solution, a solution containing stabilized α-thrombin, and a liquid fibrinogen assay reagent containing the solution. The method for stabilizing α-thrombin in a thrombin-containing solution, which includes adjusting the percentage of α-thrombin to 70% or more with respect to the amount of total thrombin in the thrombin-containing solution.