1. A method for determining abnormal gait comprising the following steps: (a) measuring ground reaction force generated during walking by a plurality of sensors arranged on a left-foot and a right-foot of a walker respectively; (b) applying measurement values measured by each of the plurality of sensors to a predetermined fuzzy membership function to transform the measurement values into first fuzzy values; (c) applying the first fuzzy values to a predetermined fuzzy logic to generate second fuzzy values corresponding to a plurality of gait phases; and (d) comparing the second fuzzy values with pre-registered data about normal gait to determine whether the walker's gait is abnormal or not.