To automatically correct a watch built in a personal computer, on the basis of the time data of signals sent from the global positioning system satellite.

A host computer 103 compares a time available from signals sent from a GPS satellite 101 with a time available from a built-in watch. The host computer 103 automatically corrects an error, if any, and synchronizes the built-in watch thereof with the time of the watch of the GPS by a second unit. Furthermore, a plurality of personal computers 107 connected to a network are capable of automatically correcting the built-in watches thereof through synchronization with the watch of the host computer 103 via a communication line.