The LSI design apparatus adds diagnostic circuitry for micro diagnosis to a behavior level description. Based on behavior level design data for a normal system of a LSI, high level synthesis generates RTL design data and register information. Based on the register information, a unique address used by a micro diagnosis program is allocated to each register. Circuit components within the normal system are grouped together. Based on the result of the address allocation and the result of the grouping, a behavior description for diagnostic circuits constituting a diagnosis system for the LSI is generated and added to the behavior level design data for the normal system, resulting in behavior level design data in which the normal and diagnosis systems are integrated together.