An ATM cell format converter comprises conversion memory having a plurality of addresses for temporarily storing an ATM cell so as to perform ATM cell-format conversion. The ATM cell is separated into a plurality of data blocks and written into the addresses of the conversion memory together with indicators including a first indicator for indicating the beginning one of the data blocks in the ATM cell and a plurality of second indicators for indicating the other data blocks which follow the beginning data block in the ATM cell. The stored data blocks are read out from the conversion memory to form a converted ATM cell having, as the beginning data block in the converted ATM cell, one of the data blocks read out together with the first indicator. Alternatively, the indicators may be stored in another memory having a plurality of addresses corresponding to the addresses of the conversion memory.