In order to enable to control a transmission data amount within an appropriate range by an easier method, and achieve throughput improvement, shortening of delay, and suppression of packet retransmission in all communication resources, a data transmission method includes: estimating a congestion degree of at least a part of communication sessions in which a communication resource to be used in an own session to be used in transmission of transmission data is shared, based on communication quality being measurable a data transmission device and relating to the transmission of the transmission data, determining a transmission data amount, based on the congestion degree of the communication session, and performing the transmission of the transmission data, based on the transmission data amount.