In a data reception processor (210a) in a session relaying apparatus (20-3), a data amount monitor (201) monitors the data amount within the session relaying apparatus (20-3), a receivable amount calculating unit (212) requires a data receivable amount which is informed from the session relaying apparatus (20-3) to a data transmission terminal (10) in a devaluating way according to a predetermined function, a receivable amount informing unit (213) informs the data transmission terminal (10) of the value, and in the data output processor (220a), an initialization judging unit (223) makes a judgment about initialization of the transmission amount, according to the amount resulting from the data monitor (201) monitoring the data amount within the session relaying apparatus (20-3), and a transmissive amount calculating unit (222) determines the data transmissive amount by using the result.