An electrically heated catalyst comprising: a heating element that heats by electrical conduction, a case that houses the said heating element, an electrode that is in contact with the said heating element, the electrically heated catalyst being characterised in that it further comprises: an inner pipe located between the said heating element and the said case that provides electrical insulation, an inner mat (51, 52) disposed in a compressed condition between the said heating element and the said inner pipe that together with providing electrical insulation supports the said heating element and that is shorter in the direction of the flow of exhaust gasses than the said inner pipe, an outer mat (53, 54) disposed in a compressed condition between the said inner pipe and the said case that together with providing electrical insulation supports the said inner pipe and that is shorter in the direction of the flow of exhaust gasses than the said inner pipe, and, wherein the compression ratio of said outer mat (53, 54) is larger than the compression ratio of said inner mat (51, 52), and said outer mat (53, 54) is shorter in the direction of the flow of exhaust gasses than said inner mat (51, 52), at both upstream and downstream of the electrode in the direction of the flow of exhaust gasses. a heating element that heats by electrical conduction, a case that houses the said heating element, an electrode that is in contact with the said heating element, the electrically heated catalyst being characterised in that it further comprises: an inner pipe located between the said heating element and the said case that provides electrical insulation, an inner mat (51, 52) disposed in a compressed condition between the said heating element and the said inner pipe that together with providing electrical insulation supports the said heating element and that is shorter in the direction of the flow of exhaust gasses than the said inner pipe, an outer mat (53, 54) disposed in a compressed condition between the said inner pipe and the said case that together with providing electrical insulation supports the said inner pipe and that is shorter in the direction of the flow of exhaust gasses than the said inner pipe, and, an inner pipe located between the said heating element and the said case that provides electrical insulation, an inner mat (51, 52) disposed in a compressed condition between the said heating element and the said inner pipe that together with providing electrical insulation supports the said heating element and that is shorter in the direction of the flow of exhaust gasses than the said inner pipe, an outer mat (53, 54) disposed in a compressed condition between the said inner pipe and the said case that together with providing electrical insulation supports the said inner pipe and that is shorter in the direction of the flow of exhaust gasses than the said inner pipe, and, the compression ratio of said outer mat (53, 54) is larger than the compression ratio of said inner mat (51, 52), and said outer mat (53, 54) is shorter in the direction of the flow of exhaust gasses than said inner mat (51, 52), at both upstream and downstream of the electrode in the direction of the flow of exhaust gasses.