A method of purifying an exhaust gas, includes: disposing a NOx trapping catalyst in an exhaust pipe of an internal combustion engine, the NOx trapping catalyst including: a metal substrate including cells, a corner portion of each of cells having an acute angle; and a catalyst layer supported in the metal substrate and including a noble metal, a heat-resistant inorganic oxide and a NOx trapping material, the catalyst layer having pores formed by addition of a pore formation promoting material, and the NOx trapping catalyst: adsorbing NOx in the exhaust gas when an exhaust air-fuel ratio is in a lean state; and desorbing and reducing the adsorbed NOx when the exhaust air-fuel ratio is in a stoichiometric state or a rich state; and removing the NOx by the exhaust air-fuel ratio being shifted between the lean state and the rich state.