In order to obtain the probable error of Loran lines of position measured by a number of ships at sea, the present writer distributed to each of the ships a printed form to be filled with observed errors. The method of obtaining error is as follows: Estimating the position fixed by cross beraring or Radar at coasting and the position fixed from astronomical observation in ocean respectively as the correct ship's position, the error is represented by the length of a perpendicular drawn from the above-mentioned position to Loran LOP. Here, the error of Loran LOP can be expressed as formula (1) of the present article. Let a…geometrically non-variable error [numerical formula]…error on the baseline due to the error of the measurement of time difference. △t…error of the measurement of time difference (μ.s.) φ…the angle between the two stations from the observer cosecφ/2…the factor of geometrical precision. Thus, if from the deta collected from the ships a number of observed errors (e) and their angles (φ) be found, then substituting each of them in formula (1), the values of a^2 and b^2 can be odtained by the method of least squares. Transforming them into probable errors, formula (5) and (6) are obtained. The results of the calculation are shown in Table 1, and they are, expressed graphically as curve B in Fig I and II. Besides, curve A in Fig I and II represent the probable error of LOP, which was estimated from an American literature.