1. A routing analysis method for performing a routing analysis on an integrated circuit from a netlist which is described hierarchically, the integrated circuit having a first hierarchical module including a second hierarchical module positioned in a lower hierarchy than the first module on the netlist, the method comprising the steps of: calculating a first routing difficulty index for a whole circuit of the first hierarchical module including the second hierarchical module; calculating a second routing difficulty index for whole circuit of the second hierarchical module; and calculating a third routing difficulty index for a circuit portion of the first hierarchical module not including the second hierarchical module using the first and second routing difficulty indices, wherein the netlist has information on cells constituting the first hierarchical module and the second hierarchical module, and routes connecting the plurality of cells, wherein the step of calculating the first routing difficulty index includes the step of obtaining a sum of areas of the cells constituting the first hierarchical module, a number of the cells constituting the first hierarchical module or a number of said routes connecting the cells constituting the first hierarchical module from said netlist, to be defined as a first constant, wherein the step of calculating the second routing difficulty index includes the step of obtaining a sum of areas of the cells constituting the second hierarchical module, a number of the cells constituting the second hierarchical module or a number of the routes connecting the cells constituting the second hierarchical module from said netlist, to be defined as a second constant, wherein the step of calculating the third routing difficulty index includes the steps of obtaining a first value which is a result of multiplying the first routing difficulty index by a first weighting factor and obtaining a second value which is a result of multiplying the second routing difficulty index by a second weighting factor, and wherein at least one of calculating steps is performed by a processor of a computer processing device.