PURPOSE: To enable surveying of a ship speed without a time lag at an arbitrary point of time, by a method wherein, using a first-in first-out system memory, a ship speed is computed from a value which is most early written in the memory and a value which is written lastly.

CONSTITUTION: Using a first-in first-out system memory FIFO, a latitude value and a longitude value, into which a hyperbolic data is converted by a centralized processing unit CPU, are written in the memory FIFO each time said conversion is made, and a reading of the latitude value and longitude value, which are most early written, and the latitude value and longitude value, which are written lastly, is obtained by the CPU. Through computatin of a distance between spots decided by each latitude value and longitude value, a ship speed can be found. This permits survey of a ship speed at the same period as a period which coordinate-converts the hyperbolic data into a latitude and a longitude, and enables an error to be enoughly reduced through arbitrary lengthening of a distance between two spots from which a ship speed is found.