  An inequality of Hardy type is established for quadratic forms involving
Dirac operator and a weight $r^{-b}$ for functions in $\R^n$. The exact Hardy
constant $c_b=c_b(n)$ is found and generalized minimizers are given. The
constant $c_b$ vanishes on a countable set of $b$, which extends the known case
$n=2$, $b=0$ which corresponds to the trivial Hardy inequality in $\R^2$.
Analogous inequalities are proved in the case $c_b=0$ under constraints and,
with error terms, for a bounded domain.
