A detailed and reliable electron collision cross section set of Cl2 gas is constructed. The present cross section set consists of one elastic momentum transfer, three vibrational excitation, eight dissociative electronic excitation, two electronic excitation, one ion-pair formation, one dissociative electron attachment, and four ionization cross sections. The electron drift velocity, longitudinal diffusion coefficient, ionization coefficient, and electron attachment coefficient in both Cl2 gas and Cl2/N2 mixtures and the electron attachment rate coefficient in mixtures of N2 gas and a minute amount of Cl2 gas are calculated by Monte Carlo simulation using the present cross section set, and the reliability of the present cross section set is demonstrated by comparing calculated and measured electron transport coefficients and rate coefficients.