An object of the present invention is to find a drug which is useful for prevention of postoperative astigmatism. The contact blades are pivotally mounted to the housing by pivot rods. The concentration of the nucleotide synthesis inhibitor in the blood of the mammal is observed and it is determined whether the nucleotide synthesis inhibitor has a half-life which is shorter than that of N–2-cyano-3-hydroxycrotonamide.