1. An encoding/decoding method of a motion image signal, wherein, in an encoder, a differenc between an orthogonal transformation coefficient obtained by orthogonally transforming an input image signal of a current frame and a current frame transformation coefficient predicted by the orthogonal transformation coefficient of the previous frame (i.e., the latter coefficient is a predicted orthogonal transformation coefficient in the encoder) is calculated, the difference signal is added to the predicted orthogonal transformation coefficient to locally decode the current frame orthogonal transformation coefficient so that a sum is used to predict an orthogonal transformation coefficient of the next frame, and the difference signal is sent as a coded image signal to a decoder; while the orthogonal transformation coefficient is coded by interframe coding, orthogonal transformation coefficients to be omitted are determined on the basis of the input image signal of the current frame and then omitted, and the resultant information as mode information is sent to the decoder; and wherein, in the decoder, the coded image signal from the encoder is added to the current frame orthogonal transformation coefficient predicted from the decoded orthogonal transformation coefficient of the previous frame, i.e., the predicted orthogonal transformation coefficient in the decoder to decode the orthogonal transformation coefficient of the current frame, and the decoded orthogonal transformation coefficient of the current frame or the predicted orthogonal transformation coefficient in the decoder is transformed according to inverse transformation and the transformed coefficient is output as a decoded image signal; while coefficient omissions are performed for the predicted transformation coefficient on the basis of the mode information from the encoder.