Encoder, encoding method, decoder, and decoding method

An encoder that encodes a first current block and a second current block in a picture, the encoder comprising:

• circuitry;
• and
• memory, wherein
• using the memory, the circuitry:
• performs a first transform on (i) a first residual signal of the first current block using a first transform basis and (ii) a second residual signal of the second current block using a second transform basis, to generate first transform coefficients;
• performs a second transform on the first transform coefficients of the first current block using a third transform basis to generate second transform coefficients and quantizes the second transform coefficients when the first transform basis used for the first transform on the first residual signal is same as a predetermined transform basis;
• and
• quantizes the first transform coefficients of the second current block without performing the second transform when the second transform basis used for the first transform on the second residual signal is different from the predetermined transform basis.