  We reveal the universality of short-term anisotropic inflation. As a
demonstration, we study inflation with an exponential type gauge kinetic
function which is ubiquitous in models obtained by dimensional reduction from
higher dimensional fundamental theory. It turns out that an anisotropic
inflation universally takes place in the later stage of conventional inflation.
Remarkably, we find that primordial gravitational waves with a peak amplitude
around $10^{-26}$ ~ $10^{-27}$ are copiously produced in high-frequency bands
10MHz~100MHz. If we could detect such gravitational waves in future, we would
be able to probe higher dimensional fundamental theory.
