A semiconductor storage device includes a first memory die. The first memory die includes a first memory plane including a plurality of first memory blocks, a second memory plane including a plurality of second memory blocks, a first sequencer, and a second sequencer. The first sequencer is configured to start a first write sequence with respect to one of the first memory blocks in response to a first command set designating the one of the first memory blocks if no write sequence is being performed by the first sequencer. The second sequencer is configured to start a second write sequence with respect to one of the second memory blocks in response to a second command set designating the one of the second memory blocks if the first sequencer is performing the first write sequence and no write sequence is being performed by the second sequencer.