Abstract: A fault current detection device which, in order to take into consideration a record in the past of a load current up to a time instance when a short-circuit is caused, operates, every sampling period .DELTA.t, a variable .DELTA.I.sub.n =f(I.sub.n, J.sub.n-1) representatve of the record obtainable as a function of an instantaneous current value I.sub.n at nth sampling time and a parameter J.sub.n-1 obtained by weighting the load current within a preceding specified time period is featured by that the parameter J.sub.n-1 is substituted by the present current value I.sub.n when a variation of current is in a decreasing direction and that a variable .DELTA.I.sub.n =f(D.sub.n, .DELTA.I.sub.n-1) obtainable as a function of a current varying rate D.sub.n at nth sampling time and a variable .DELTA.I.sub.n-1 representative of a current varying rate at (n-1)th sampling time is calculated every sampling period .DELTA.t with variable .DELTA.I.sub.n being zero when D.sub.n is negative.