There is provided a field effect transistor having, on a substrate, at least a gate electrode, a gate insulating film, an active layer mainly containing an oxide semiconductor that contains at least one of In, Ga or Zn, a source electrode, and a drain electrode, the field effect transistor including: a heat diffusion layer, wherein, given that a thermal conductivity of the substrate is Nsub (W/mK), a thermal conductivity of the heat diffusion layer is Nkaku (W/mK), a film thickness of the heat diffusion layer is T (mm), a planar opening ratio of the heat diffusion layer is R (0≦R≦1), and S=T×R, the thermal conductivity Nsub of the substrate satisfies the condition Nsub 3.0×S^(−0.97×e^(−1.2×Nsub)) and Nkaku≧Nsub.