M. Hoyanagi pointed out a position deviation of Kyushu Island on an Inoh map with a scale of 1 to 216000. He explained it referring to the map projection adopted by Inoh and the magnetic declination at the beginning of 19th century. In this study, the correlation between the coordinate value of an astronomical observation point on the map and the corresponding location (latitude and longitude) on the actual map is statistically analyzed. The coefficients of determination (R2) are calculated for maps rotated by small degrees. As a result, anticlockwise rotation of the map by 0.975 degree gave the max value of R2 (0.9995). This value corresponds to a magnetic declination of about 1.0 degree to the west at that time in the central region of Kyushu Island. It is concluded that the position deviation on the map is caused by magnetic declination, because Inoh considered magnetic north to be true north when surveying and mapping.