1. A circuit design device for deciding placement of elements and interconnections included in a circuit, on a basis of connection information of the circuit, the circuit design device comprising: an equivalent fault class extracting unit for extracting one or more equivalent fault classes each of which comprises a class having, as members, equivalent fault interconnections that comprises interconnections mutually causing an equivalent fault in the circuit; a weighting unit for giving a greater weight to the equivalent fault class or the equivalent fault interconnections included in the equivalent fault class, as a number of the equivalent fault interconnections that is a number of the members in the equivalent fault class increases; and a placement deciding unit for deciding placement of the elements and the interconnections so that, among the equivalent fault classes, an equivalent fault class having a larger number of equivalent fault interconnections would have a lower fault probability that is a probability of including a single stuck-at fault.