Encoder, decoder, encoding method, and decoding method

A decoder which obtains a bitstream in which image information has been encoded and decodes the image information, the decoder comprising:

• memory;
• circuitry accessible to the memory,
• wherein the circuitry accessible to the memory:
• obtains the bitstream which includes a binary data string in which the image information has been binarized;
• switches whether or not to apply arithmetic decoding to the binary data string included in the bitstream;
• when arithmetic decoding is applied to the binary data string, applies arithmetic decoding to the binary data string, and inverse binarizes the binary data string to which arithmetic decoding has been applied, to decode the image information;
• when arithmetic decoding is not applied to the binary data string, skips applying arithmetic decoding to the binary data string, and inverse binarizes the binary data string to which arithmetic decoding has not been applied, to decode the image information;
• when inverse binarizing the binary data string, inverse binarizes a first portion which is included in the binary data string and in which frequency transform coefficient information related to frequency components of an image has been binarized, according to different inverse binarization formats between when arithmetic decoding is applied to the binary data string and when arithmetic decoding is not applied to the binary data string;
• and
• when inverse binarizing the binary data string, inverse binarizes a second portion which is included in the binary data string and in which a part or an entirety of prediction parameter information related to prediction methods of an image has been binarized, according to a common inverse binarization format between when arithmetic decoding is applied to the binary data string and when arithmetic decoding is not applied to the binary data string,
• wherein the circuitry inverse binarizes the second portion in which at least one of the following kinds of information included in the prediction parameter information has been binarized:
• information related to coding modes;
• information related to prediction directions in intra prediction;
• information related to reference pictures in inter prediction;
• and information related to motion vectors in inter prediction, according to the common inverse binarization format between when arithmetic decoding is applied to the binary data string and when arithmetic decoding is not applied to the binary data string.