A thermoplastic elastomer composition obtainable by dynamically crosslinking an ethylene/α-olefin/non-conjugated polyene copolymer, a polyolefin resin, a softener in an amount of 1 to 200 parts by mass per 100 parts by mass of the total of the copolymer and the resin, and a crosslinking agent, by the use of a batch mixer having intermeshing rotors under the conditions satisfying the following requirements to: 50≤P1≤300 wherein P1 is defined by the following formula (i): [Math. 1] P 1 = γ ⁢ ave. × Log t 1 wherein the residence time (sec) after the introduction of the crosslinking agent is denoted as t1 sec, and the average rate (sec -1) of shear rates γ in the t1 is denoted as γave. sec -1, 900≤P2≤1450 wherein P2 is defined by the following formula (ii): [Math. 2] P 2 = Tave. + 273 × Log t 1 wherein the average resin temperature (°C) after the introduction of the crosslinking agent is denoted as Tave. °C, and the residence time (sec) after the introduction of the crosslinking agent is denoted as t1 sec, -2.5≤P3≤1.5 wherein P3 is defined by the following formula (iii): [Math. 3] P 3 = Log t 1 / t 2 wherein the residence time (sec) after the introduction of the crosslinking agent is denoted as t1 sec, and the time (sec) required for the residual amount of the crosslinking agent to become not more than 1 ppm of the introduction amount thereof at the average resin temperature Tave. after the introduction of the crosslinking agent is denoted as t2 sec, and the crosslinking agent is introduced into the batch mixer after the ethylene/α-olefin/non-conjugated polyene copolymer, the polyolefin resin and the softener have been introduced and the softener has been kneaded with the ethylene/α-olefin/non-conjugated polyene copolymer and the polyolefin resin, wherein the ethylene/α-olefin/non-conjugated polyene copolymer and the polyolefin resin are contained in a mass ratio ( /) of 90/10 to 10/90, and the crosslinking agent is an organic peroxide, and the amount of the crosslinking agent compounded is 0.01 to 3.0 parts by mass per 100 parts by mass of the total of the copolymer and the resin. 50≤P1≤300 wherein P1 is defined by the following formula (i): [Math. 1] P 1 = γ ⁢ ave. × Log t 1 wherein the residence time (sec) after the introduction of the crosslinking agent is denoted as t1 sec, and the average rate (sec -1) of shear rates γ in the t1 is denoted as γave. sec -1, 900≤P2≤1450 wherein P2 is defined by the following formula (ii): [Math. 2] P 2 = Tave. + 273 × Log t 1 wherein the average resin temperature (°C) after the introduction of the crosslinking agent is denoted as Tave. °C, and the residence time (sec) after the introduction of the crosslinking agent is denoted as t1 sec, -2.5≤P3≤1.5 wherein P3 is defined by the following formula (iii): [Math. 3] P 3 = Log t 1 / t 2 wherein the residence time (sec) after the introduction of the crosslinking agent is denoted as t1 sec, and the time (sec) required for the residual amount of the crosslinking agent to become not more than 1 ppm of the introduction amount thereof at the average resin temperature Tave. after the introduction of the crosslinking agent is denoted as t2 sec, and the crosslinking agent is introduced into the batch mixer after the ethylene/α-olefin/non-conjugated polyene copolymer, the polyolefin resin and the softener have been introduced and the softener has been kneaded with the ethylene/α-olefin/non-conjugated polyene copolymer and the polyolefin resin, wherein the ethylene/α-olefin/non-conjugated polyene copolymer and the polyolefin resin are contained in a mass ratio ( /) of 90/10 to 10/90, and the crosslinking agent is an organic peroxide, and the amount of the crosslinking agent compounded is 0.01 to 3.0 parts by mass per 100 parts by mass of the total of the copolymer and the resin.