In conventional thermonuclear fusion process of atomic nuclei, one critical condition is how to make them close together or pass through the Coulomb barrier between themselves. Although the thermonuclear fusion has been understood well with the conventional theory in nuclear physics, cold fusion, which has been issued with Fleischmann-Pons experiment in 1989, or nuclear fusion at room temperature in general is not explained with the conventional theory. However, if nuclear force inside the nucleus is considered as the spin-spin magnetic interaction of nucleons initiated by electric polarization between proton and neutron, there is an alternative way to understand cold fusion. Since the electric polarization between nucleons can be induced by an external electric field, the electric polarization can be an important clue to understand cold fusion, such as the nuclear fusion of deuterium nuclei absorbed into the lattice structure of palladium.