PURPOSE: To omit the unnecessary processing and communication of a computer and a terminal unit, to make their load light and to decrease the communication expense in the data communication system between the computer and the terminal unit.

CONSTITUTION: The data communication system is constructed to connect a computer 10 and a terminal unit 20 with a communication line 30. The computer 10 has a business operation date transmitting part 11. The business operation date transmitting part 11 can store the data of each business operation date beforehand and can send this. The terminal unit 20 has a business operation date receiving part 21, a business operation date storing part 22 and a business operation date confirming part 23. The business operation date confirming part 23, when the input of business data is made, retrieves the business operation date storing part 22, confirms whether or not the business of the inputted business data can be operated, when it can be operated, transmits the business data to the computer 10, and when it cannot be operated, does not transmit them and informs an operator that the operation date is not correct.