The characteristics of Loran sky wave propagation errors is very important in order to determine the probable errors of Loran fix using the sky wave. These errors near the Japan area are evaluated by measuring the Loran sky wave time differences from each Loran station to the fixed receiving position and compared with the Pierce's equation. Observed results are shown in the figures representing the relation between the relative number of observation and the sky wave reading. By Pierce's equation, the probable errors of sky wave delay are depend on the distance between the Loran station and the receiving position. So the errors of sky wave reading depend upon the both distances from the master and slave station to the receiving position. We propose the equivalent distance by which the errors are decided. The errors, however, are also the function of the base line length. We can decide these errors by the equivalent distance and the base line length. The probable errors of Loran sky wave propagation calculated from the observed results are shown in the figures as the functions of the equivalent distance and the base line length, and one can see that these results are considerably agree with the Pierce's equation.