  We investigate the Kondo effect in Dirac systems, where Dirac electrons
interact with the localized spin via the s-d exchange coupling. The Dirac
electron in solid state has the linear dispersion and is described typically by
the Hamiltonian such as $H_k= v{\bf k}\cdot {\sigma}$ for the wave number ${\bf
k}$ where $\sigma_j$ are Pauli matrices. We derived the formula of the Kondo
temperature $T_{\rm K}$ by means of the Green's function theory for small $J$.
The $T_{\rm K}$ is determined from a singularity of Green's functions in the
form $T_{\rm K}\simeq \bar{D}\exp(-{\rm const.}/\rho |J|)$ when the exchange
coupling $|J|$ is small where $\bar{D}=D/\sqrt{1+D^2/(2\mu)^2}$ for a cutoff
$D$ and $\rho$ is the density of states at the Fermi surface. When $|\mu|\ll
D$, $T_{\rm K}$ is proportional to $|\mu|$: $T_{\rm K}\simeq |\mu|\exp(-{\rm
const.}/\rho |J|)$. The Kondo screening will, however, disappear when the Fermi
surface shrinks to a point called the Dirac point, that is, $T_{\rm K}$
vanishes when the chemical potential $\mu$ is just at the Dirac point. The
resistivity and the specific heat exhibit a log-$T$ singularity in the range
$T_{\rm K} < T\ll |\mu|/k_{\rm B}$. Instead, for $T\sim O(|\mu|)$ or $T>|\mu|$,
they never show log-$T$.
