  The Ru doping effect on the Dirac cone states is investigated in iron
pnictide superconductors Ba(Fe$_{1-x}$Ru$_x$As)$_2$ using the transverse
magnetoresistance (MR) measurements as a function of temperature. The linear
development of MR against magnetic field $B$ is observed for $x$ = 0 - 0.244 at
low temperatures below the antiferromagnetic transition. The $B$-linear MR is
interpreted in terms of the quantum limit of the Dirac cone states by using the
model proposed by Abrikosov. An intriguing evidence is shown that the Dirac
cone state persists on the electronic phase diagram where the
antiferromagnetism and the superconductivity coexist.
