To eliminate the need for an external clock and a communication means with Internet when a time is synchronized between a master station and a slave station.

When the first correction time is inputted, the current time in a computer is stored as the first correction time (S2). A period na required for generating an error of 1 sec in the computer is obtained from a time difference t between the first correction time and the second correction time, and an error s of the current time in the computer (S4). A process for correcting a clock in the computer by 1 sec at the correction period na is repeated (S6, S8). At the third correction time, a correction period za is obtained by adjusting the correction value at the correction period na in the second correction by the period na required for generating the error of 1 sec in the computer. A process for correcting the clock in the computer is repeated at each correction period za (S7).