A mesh-like connecting member 25 (a connecting member) is provided in the back gate electrode film 13. As shown in FIG. 3A, the shape of the mesh-like connecting member 25 is a mesh shape in which a plurality of through-holes 26 are periodically formed in a flat plate. The through-holes 26 are arrayed in zigzag on an XY plane. A part of the back gate electrode film 13 enters the insides of the through-holes 26. In other words, in the back gate electrode film 13, column sections 13a that pierce through the mesh-like connecting member 25 are provided. As shown in FIG. 2, the mesh-like connecting member 25 is divided into an upper layer portion 25a and a lower layer portion 25b. An air gap 25c is formed between the upper layer portion 25a and the lower layer portion 25b. The air gap 25c may be formed over the entire surface or may be partially formed. The mesh-like connecting member 25 may be a continuum. The air gap 25c does not have to be formed.
