We propose a numerical method for analyzing near-field magnetic fields for automobile electronic key systems. The surface of an automobile body is divided into triangular patches and a modified inductance matrix formulation is employed. A large inductance matrix equation is solved using a block Gauss-Seidel method. A source magnetic field produced by a ferrite bar antenna is analyzed using a magnetic moment method to formulate a right-hand vector of the matrix equation. We have made a simplified automobile model and measured leakage of magnetic fields from windows of the model. Analyzed results agreed with the measured results with an error of 10%.