An encoder includes: circuitry; and memory. Using the memory, the circuitry: performs, when a size of a current block to be subjected to transform processing is not a power of 2, complementary processing of adding a complementary region to the current block to cause the size to be a power of 2; performs transform processing on the current block which has been subjected to the complementary processing; performs inverse transform processing on the current block which has been subjected to the transform processing; and eliminates the complementary region included in the current block which has been subjected to the inverse transform processing.