The importance of the far-infrared (FIR) mapping is demonstrated for a face-on spiral galaxy, M81, by analysing its imaging data at 65, 90 and taken by AKARI. The basic products are the dust temperature map, the dust optical depth map and the colour–colour diagram. The main features are as follows. (i) The dust temperature derived from the total fluxes at 90 and 140 reflects the relatively low temperatures seen in the interarm and spiral arms excluding the warm spots rather than the high temperatures in warm spots and the centre. This indicates that the total FIR luminosity is dominated by the dust heated by the general interstellar radiation field. (ii) The galaxy is more extended at than at the other shorter wavelengths, which reflects the radial dust temperature gradient. (iii) The dust optical depth derived from the FIR mapping is broadly consistent with that estimated from the FIR-to-ultraviolet luminosity ratio. (iv) The FIR colour–colour diagram is useful to identify a ‘contamination’ of warm dust. The existence of small-scale warm star-forming regions is supported in the bright spots along the spiral arms. This contamination also leads to an underestimate of dust optical depth (or dust column density).