To provide an image encoder which can reduce increase in processing amount in case of rule violation.

The image encoder comprises a predictive encoder 10 for generating encoded data by predictively encoding image data, a binarizer 21 for generating binary data by binarizing the encoded data for arithmetic encoding, a buffer 22 for holding the binary data, an arithmetic encoder 23 arithmetically encoding binary data held in the binary data buffer 22 to generate stream data and operating in asynchronism with the binarizer 21; and a detector 30 for detecting the stream data containing macro blocks of code amount more than a predetermined threshold wherein the arithmetic encoder 23 arithmetically encoding the binary data held in the binary data buffer 22 successively when stream data containing macro blocks of code amount more than the predetermined threshold are detected at the detector 30.