  We analyse the origin of the magnetic field decay in normal radio pulsars
found by us in a recent study. This decay has a typical time scale $\sim 4
\times 10^5$~yrs, and operates in the range $\sim 10^5$~--~few$\times
10^5$~yrs. We demonstrate that this field evolution may be either due to the
Ohmic decay related to the scattering from phonons, or due to the Hall cascade
which reaches the Hall attractor. According to our analysis the first
possibility seems to be more reliable. So, we attribute the discovered field
decay mainly to the Ohmic decay on phonons which is saturated at the age
few$\times 10^5$~yrs, when a NS cools down to the critical temperature below
which the phonon scattering does not contribute much to the resistivity of the
crust. Some role of the Hall effect and attractor is not excluded, and will be
analysed in our further studies.
