The map projections used in Japan at the time of Inoh's map are studied, and the graticule and map projection of his map are examined on the basis of research to date. As many researchers have already pointed out, a contradiction exists between the map projection of Inoh's map and the graticule drawn on it. Ohtani (1917) concludes that the graticule on Inoh's map was drawn using the Sanson–Flamsteed projection and Hoyanagi (1974) concurs that this was certainly the projection of Inoh's map. On the other hand, Unno (1985b) disagrees that the Sanson–Flamsteed projection was used, and instead identifies a trapezoidal projection. Ohtani (1917) also introduces the map-making technology of Inoh Tadataka in detail. Using his technology, survey results are expanded on a plane without modification, and no conversion from a spherical surface to a plane is performed. As a result of verifying the graticule on Inoh's map and his map projection, it is highly probably that the graticule on Inoh's map was drawn with a trapezoidal projection, and it is proved that Inoh's map corresponds well to an equidistant secant cylindrical projection. However, the standard parallels of these equidistant secant cylindrical projection maps vary on every map.