A technique is proposed for reducing leakage electromagnetic field that reduces the electromagnetic leakage from a connection point between a communication coupler and a signal transmission apparatus and that can conduct both communication and transmission of electric power. The signal transmission apparatus includes: a first sheet conductive body portion constituting a lower portion electrode; and a second sheet conductive body portion in a mesh shape which is laminated on the first sheet conductive body portion while a gap area is sandwiched therebetween and which includes an uneven mesh size area on which a sheet impedance is continuously changed.