We estimated the scale-length of the thin disc with the J and W1 magnitudes of the most probable Red Clump (RC) stars in the Galactic plane, −0.5 deg.<b<+0.5 deg., in 19 equal sized fields with consecutive Galactic longitudes which cover the interval 90 deg.<l<270 deg. Our results are constrained with respect to the solar space density (D*=5.95), which indicates that the radial variation of the density is lower for higher Galactocentric distances. The scale-length of the thin disc is 2 kpc for the fields in the Galactic anticentre direction or close to this direction, while it decreases continuously in the second and third quadrants reaching to a lower limit of h=1.6 kpc at the Galactic longitudes l=90 deg. and l=270 deg. The distribution of the scale-length in 19 fields is consistent with the predictions from the "Galaxia" model and its variation with longitude is probably due to the inhomogeneity structure of the disc caused by the accreted material or other features such as warp and flare.