The superconducting energy gap of heavily doped Re thin films has been measured by electron tunneling of Re/Al2O3/Al junctions. We get △0= (1.04±0.02) meV, 2△0/KTc = 3.31±0.04. The △0 value is determined by the maximum conductance method. It is shown that the energy gap and the Tc of Re films increases many times by impurity, but Re remains a weakly coupled superconductor.