In a network-type data transmission system wherein a server 4(5) and a plurality of terminal units (client computers) 1(2,3) sharing the server 4(5) are connected by way of a network circuit 6, a packet generating means 223 of each terminal unit generates a packet command complying with a DB command transmission protocol independent of the network circuit each time the terminal unit collects data, and a packet converting means 423 of the server having received the packet command converts the packet command into a DB operation command, executes the DB operation command, and reflects an executed result on the DB, so that a demand from the terminal unit is responded in real time.With the above structure, an operation of a database server from the terminal unit is realized only by implementing a communication driver which realizes the TCP-IP, and an effective sales management system such as the POS system can be constructed.