  Motion of a particle near a horizon of a spherically symmetric black hole is
shown to possess a universal Lyapunov exponent of a chaos provided by its
surface gravity. To probe the horizon, we introduce electromagnetic or scalar
force to the particle so that it does not fall into the horizon. There appears
an unstable maximum of the total potential where the evaluated maximal Lyapunov
exponent is found to be independent of the external forces and the particle
mass. The Lyapunov exponent is universally given by the surface gravity of the
black hole. Unless there are other sources of a chaos, the Lyapunov exponent is
subject to an inequality $\lambda \leq 2\pi T_{\rm BH}/\hbar$, which is
identical to the bound recently discovered by Maldacena, Shenker and Stanford.
