To provide a gate insulating material which has high chemical resistance, is superior in coatability of a resist and an organic semiconductor coating liquid, and has small hysteresis, a gate insulating film and an FET using the same by a polysiloxane having an epoxy group-containing silane compound as a copolymerization component.A gate insulating material containing a polysiloxane having, as copolymerization components, at least a silane compound represented by the general formula (1): R1mSi(OR2)4-m  (1), wherein R1 represents hydrogen, an alkyl group, a cycloalkyl group, a heterocyclic group, an aryl group, a heteroaryl group or an alkenyl group and in the case where a plurality of R1s are present, R1s may be the same or different, R2 represents an alkyl group or a cycloalkyl group and in the case where a plurality of R2s are present, R2s may be the same or different, and m represents an integer of 1 to 3, and an epoxy group-containing silane compound represented by the general formula (2): R3nR4lSi(OR5)4-n-1  (2), wherein R3 represents an alkyl group or a cycloalkyl group having one or more epoxy groups in a part of a chain and in the case where a plurality of R3s are present, R3s may be the same or different, R4 represents hydrogen, an alkyl group, a cycloalkyl group, a heterocyclic group, an aryl group, a heteroaryl group or an alkenyl group and in the case where a plurality of R4s are present, R4s may be the same or different, R5 represents an alkyl group or a cycloalkyl group and in the case where a plurality of R5s are present, R5s may be the same or different, l represents an integer of 0 to 2, and n represents 1 or 2 (however, l+n≦3).