To provide a simple method for analyzing the earthquake response of pile groups.

A pile circumferential friction force per unit length of a pile group is modeled into a lumped mass model 10 having mass points 11 indicating the pile group and springs 15 connected to the mass points 11. Spring constants of the springs 15 are computed by multiplying both a pile group coefficient computed on the basis of the number of piles, the intervals between the piles, and the diameters of the piles and the number of the piles by a spring constant of a spring indicating a pile circumferential friction force per unit length of a single pile. Through the use of the computed spring constants, the earthquake response of the lumped mass model 10 is analyzed.