PURPOSE: To enable correction of measuring errors caused by an ionosphere handily at a high accuracy, by calculating two base-line vectors based on frequencies of two signals transmitted from satellites.

CONSTITUTION: First and second signals SAL1, SAL2WSDL1 and SDL2 transmitted from satellites 2AW2D are received at a plurality of observing points P1 and P2. Then, first and second base-line vectors BL1 and BL2 are calculated between the observing points P1 and P2 corresponding to the first and second signals SAL1, SAL2WSDL2 and SDL2 based on phase differences ΔAL1, ΔAL2, ΔBL1 and ΔBL2 between the observing points P1 and P2 for the first and second signals SAL1, SAL2WSDL1 and SDL2. Based on frequencies fL1 and fL2 of the first and second signals SAL1, SAL2WSDL1 and ADL2, a value of the first or second base-line vector BL1 or BL2 is corrected from the first and second base- line vectors BL1 and BL2 to calculate a relative position between the observing points P1 and P2.