To solve alternating magnetic field problems, the program TDEDDY is developed with the use of the finite element method. Triangular finite elements of the first order are employed for the discretization of the field region. This program is constructed through an interactive system to check errors immediately when every routine is executed, and several subprograms are employed for the graphic representation of computed results. As an example, it is applied to a model of semi-infinite slab excited by an alternating current source. Results by the program TDEDDY show the satisfactory accuracy in comparision with those of analytic calculations.