To confirm that a proper receiver has received data sent by a proper sender and to establish evidence as a reception confirmation document for the data by allowing a transmission-side device to confirm the reception of the whole document by sending a next division piece after confirming that correct division piece reception confirmation data are received.

The transmission side sends a sender's name S, the number N of data divisions, and a message digest H(m) of data (m) to the reception side (S201). Then the reception side sends an expression I;hN(r) (h: message digest function) generated by applying a unidirectional function to a random number (r) N times, the sender's name S, a receiver's name R, the number N of data divisions, the message digest H(m) of the data (m), and a digital signature for them to the transmission side (S202). Then the transmission side sends one data mi among the divided data to the reception side (S203). At this time, the reception of the whole document is confirmed.