In order to avoid generation of a routing complexity of LSI and a signal rounding due to insertion of the isolation testing circuit, if a plurality of IPs are incorporated into LSI, the present invention provides an isolation testing circuits having test switching selectors 731 to 736 for selecting any one of a test input signal (or a test input transit signal) and a normal input signal, and test signal transit buffers 721 to 726 for relaying the test input signal (or the test input transit signal) are formed in respective IP blocks 701 to 706 incorporated into an LSI. Adjacent isolation testing circuits are connected mutually based on a floor plan or layout placement information such that a wiring length of a test input signal 709 and test input transit signals 710 to 714, which are connected in a single stroke of a pen, can be reduced shortest.