In a data reception processing device (210a) of a session relay device (20-3), a data amount monitoring unit (201) detects a data amount in the session relay device (20-3); and a reception-enabled amount calculation unit (212) underestimates the data reception enabled amount to be reported from the session relay device (20-3) to a data transmission terminal (10) according to a predetermined function. The underestimated value is reported to the data transmission terminal (10) by a reception enabled amount report unit (213). In a data output processing device (220a), according to the data amount in the session relay device (20-3) detected by the data amount monitoring unit (201), an initialization judgment unit (223) judges the initialization of the transmission amount. By using the result,a transmission- enabled amount calculation unit (222) decides the data transmission enabled amount.