Liquid ejection head, recording apparatus and heat radiation method for liquid ejection head

Provided is a liquid ejection head including: a plurality of recording element substrates including energy generating elements that generate ejection energy for ejecting liquid from ejection orifices; a first support member that supports the plurality of recording element substrates such that the recording element substrates are arranged in one or more lines on a main surface of the first support member; and a second support member that supports the first support member on a surface opposite to the main surface. A first thermal resistance concerning an in-plane direction parallel to the main surface, of a region between the recording element substrates in the first support member is higher than a second thermal resistance concerning a thickness direction of the second support member, of a projection region that overlaps with each recording element substrate in the second support member.