  It is shown that a subgroup of $SL(2,{\mathbb H})$, denoted $Spin(2,{\mathbb
H})$ in this paper, which is defined by two conditions in addition to unit
quaternionic determinant, is locally isomorphic to the restricted Lorentz
group, $L_+^\uparrow$. On the basis of the Dirac theory using the spinor group
$Spin(2,{\mathbb H})$, in which the charge conjugation transformation becomes
linear in the quaternionic Dirac spinor, it is shown that the Hermiticity
requirement of the Dirac Lagrangian, together with the persistent presence of
the Pauli-G\"ursey SU(2) group, requires an additional imaginary unit (taken to
be the ordinary one, $i$) that commutes with Hamilton's units, in the theory. A
second quantization is performed with this $i$ incorporated into the theory,
and we recover the conventional Dirac theory with an automatic
`anti-symmetrization' of the field operators. It is also pointed out that we
are naturally led to the scheme of complex quaternions, ${\mathbb H}^c$, in
which a space-time point is represented by a Hermitian quaternion, and that the
isomorphism $SL(1,{\mathbb H}^c)/Z_2\cong L_+^\uparrow$ is a direct consequence
of the fact $Spin(2,{\mathbb H})/Z_2\cong L_+^\uparrow$. Using $SL(1,{\mathbb
H}^c)\cong SL(2,{\mathbb C})$, we make explicit the Weyl spinor indices of the
spinor-quaternion, which is the Dirac spinor defined over ${\mathbb H}^c$.
