To provide an asynchronous data synchronizing method which synchronizes data occurring at the same time with each other to output them with respect to plurality of asynchronously occurring input data sequences.

CPU boards 611 and 612 consisting of CPUs 621 and 622 and RAMs 631 and 632 are provided for input data sequences of an echo sounder 1 and a GPS(global positioning system) receiver 2 respectively, and RAMs 621 and 622 are shared between CPUs 621 and 622. Each time position data from the GPS receiver 2 where the delay time to data occurrence is longer is inputted, RAMs 631 and 632 are accessed by the CPU 622 to read position data D1' of the GPS receiver 2 and water depth data D1 of the echo sounder 1 retraced by the delay time of data D1', and two read data D1 and D1' are converted into serial data of a prescribed from and are outputted.