Prediction mode selecting device in moving image coder

A moving image coder in which each of frames in a moving image sequence is divided into frame blocks and a data amount of the moving image is compressed by selecting one of a forward prediction mode, a backward prediction mode and a bidirectional prediction mode by using a motion vector for every frame block;

• the moving image coder comprising:
• a forward predicting motion vector/prediction error amount detecting section for detecting an optimum motion vector from a past frame, making a predicted image and calculating a first prediction error amount from a present frame;
• a backward prediction motion vector/prediction error amount detecting section for detecting an optimum motion vector from a future frame, making a predicted image arid calculating a second prediction error amount from the present frame;
• and
• a comparator/mode selecting section for receiving the first prediction error amount from said forward predicting motion vector/prediction error amount detecting section and the second prediction error amount from said backward predicting motion vector/prediction error amount detecting section, calculating a parameter as a function of difference between and/or ratio of the first and second prediction error amounts and selecting one of said forward prediction mode, said backward prediction mode and said bidirectional prediction mode, by comparing a value of said parameter with predetermined thresholds, so that the selecting operation is performed without calculating bidirectional prediction error amount based on the forward prediction motion vector and the backward prediction motion vector.