We have developed an algorithm called the "spectral-domain-to-real-space approach" (SDRSA) to analytically calculate radiation from the two-dimensional current density distribution in microstrip line configurations where the microstrip lines are represented in the form of a three-dimensional inhomogeneous structure. The algorithm is based on the spectral-domain approach used to estimate radiation from microstrip line configurations. Calculation results obtained by using the SDRSA and the current density distribution from a quasi-TEM mode model of microstrip lines agree well with the corresponding estimations obtained by using the equivalent electric current source method and the magnetic current source method, and with the experimental results obtained in the frequency band of up to 1 GHz.