An apparatus for designing a semiconductor integrated circuit having a configurable processor includes: a software information storage section storing software information including software descriptions; a hardware information storage section storing hardware information including hardware descriptions of the semiconductor integrated circuit; a processor information storage section storing processor information; a hardware implementation specification section specifying an algorithm of a portion to be implemented by hardware from among the software descriptions; a hardware changing section generating hardware descriptions from the algorithm of the specified portion, and changing the hardware information and the processor information; a software changing section generating software descriptions in which the algorithm of the specified portion is replaced with an algorithm for controlling the implemented hardware, and changing the software information; and a performance analyzer analyzing a performance of the semiconductor integrated circuit based on the changed software information, the changed hardware information and the changed processor information.