An overcurrent detector realized by using a computer. The apparatus is provided with an overcurrent detector (5) which calculates the temperature rise .theta.(n) =K.sub.1 P(n)+K.sub.2 .theta.(n-1), based on a current (In) flowing in an electric machine, under the conditions that the machine has an electric resistance (r), the machine works in a known ambient temperature, the allowable temperature rise (.theta.(max)) of the machine is known, a first coefficient (K.sub.1) and a second coefficient (K.sub.2) are predetermined. The overcurrent detector (5) further compares the temperature rise (.theta.(n)) with a difference between the allowable temperature rise (.theta.(max)) of the machine and the ambient temperature at which the machine is used, and determines that the machine is in an overcurrent position, when the former exceeds the latter.