A signal transmission insulating device includes: a first coil; a second coil opposing the first coil to form a transformer together with the first coil; a first insulating film provided between the opposing first coil and second coil and made of a first dielectric material; a second insulating film surrounding the first coil and made of a second dielectric material having a lower resistivity or a higher permittivity than the first dielectric material; and a third insulating film surrounding the second coil and made of a third dielectric material having a lower resistivity or a higher permittivity than the first dielectric material.