A transmission line apparatus includes: a substrate 101 with a ground conductor plane; and first and second signal strips 102a, 102b supported on the substrate 101 in parallel with each other. The apparatus further includes at least one additional capacitance element 301 that connects the first and second signal strips 102a, 102b together. The element 301 includes: a first additional conductor 303 spaced from the first signal strip 102a; a second additional conductor 305 spaced from the second signal strip 102b; and a third additional conductor 307 connected to the first and second additional conductors 303, 305 at respective points. When measured in a signal transmission direction, the smallest width W3a of the third additional conductor 307 is shorter than the length L1 or L2 of the first or second additional conductor 303 or 305. And the additional capacitance element 301 has a resonant frequency that is higher than the frequency of a signal being transmitted.