A method for preparing an isothiocyanate product from a cruciferous plant material is disclosed which is capable of stably preparing an isothiocyanate product in a sufficient amount in a high yield by subjecting a natural cruciferous plant material such as wasabi, horseradish, watercress or the like as a starting material to sufficient enzymatic reaction and, in particular, which is capable of stably preparing an isothiocyanate product containing no pungent ingredient in a sufficient amount in a high yield from a natural cruciferous plant material. A cruciferous plant material consisting of pulverized wasabi and/or horseradish was maintained at a temperature of -3.degree. C. to 50.degree.0 C. for a period of time sufficient for effecting enzymatic reaction to thereby form an isothiocyanate product containing allyl isothiocyanate as a pungent ingredient, and then the isothiocyanate product is subjected to a step of drying, distillation, extracting or combination thereof to remove the allyl isothiocyanate from the isothiocyanate product and thereby to obtain isothiocyanate product containing no pungent ingredient. Further, a cruciferous plant material consisting of pulverized watercress is maintained at a temperature of -3.degree.0 C. to 50.degree.0 C. for a period of time sufficient for effecting enzymatic reaction to thereby obtain an isothiocyanate product containing no pungent ingredient in a sufficient amount.