1. A circuit design device for deciding and changing a 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 comprise 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 decreasing a fault probability by changing the placement of the elements and the interconnections, the fault probability comprising a probability that a single stack-at fault is included in an equivalent fault class which has a larger number of equivalent fault interconnections.