To solve a problem that an operation to measure the abnormality of the resistance value of many cables is complicated.

Signal lines, in pieces, which have the same resistance are connected in series (S100), and their series connection resistance RS is measured (S102). The same signal lines in n pieces are connected in parallel with each other (S104), and their parallel connection resistance RP is measured. The ration of a resistance RS' which is obtained by dividing the resistance RS by n to a resistance RP' which is obtained by multiplying the resistance RP by n is used as an evaluation index value α (S103). On the basis of whether the index value α is substantially equal to 1 (S110), whether the resistance value of a cable is normal (S112) or abnormal (S114) is judged.