A linear analysis and 2 1/2-dimensional full-particle simulations are performed to investigate mechanisms of an electron heating due to a strong Hall current, which is caused by a large velocity difference between electrons and ions in the outflow region inside the diffusion region of the magnetic reconnection process. The numerical solution of the kinetic dispersion relation indicates that the prospective unstable mode varies according to the electron-ion relative velocity Vd; the kinetic cross-field streaming instability (KCSI) is dominant when Vd is under the substantial fraction of the electron thermal velocity ve, while the electron cyclotron drift instability (ECDI) is first excited for the case where Vd exceeds that critical velocity. We show that electrons are heated parallel to the ambient magnetic field due to the KCSI and perpendicularly due to the ECDI. In particular, electrons are very quickly heated up to a high temperature when the ECDI is dominant. However, the electron heating is ineffective when Vd/ve < 1.