1. A method for determining abnormal gait comprising: measuring, by a plurality of sensors, ground reaction force generated during walking, wherein the plurality of sensors are arranged on a left foot and a right foot of a walker; applying, by a fuzzy calculator, 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; applying, by the fuzzy calculator, the first fuzzy values to a predetermined fuzzy logic to generate second fuzzy values corresponding to a plurality of gait phases; and comparing, by a gait-state determiner, the second fuzzy values with pre-registered data about normal gait; and diagnosing an abnormal gait or a normal gait of the walker based on the comparison of the second fuzzy values with the pre-registered data and reporting the diagnosis to a practitioner, wherein the fuzzy membership function is defined as f Large ⁡ ( x ) = 1 2 ⁢ tanh ⁡ ( s ⁡ ( x - x 0 ) ) + 1 ∈ [ 0 , 1 ] f Small ⁡ ( x ) = 1 - f Large ⁡ ( x ) ∈ [ 0 , 1 ] and, wherein f Large (x) and f Small (x) are the first fuzzy values, x is a measurement value, x 0 is a predetermined reference value, and s is a sensibility coefficient.