  We study excited states of the nucleon in quenched lattice QCD with the
spectral analysis using the maximum entropy method. Our simulations are
performed on three lattice sizes $16^3\times 32$, $24^3\times 32$ and
$32^3\times 32$, at $\beta=6.0$ to address the finite volume issue. We find a
significant finite volume effect on the mass of the Roper resonance for light
quark masses. After removing this systematic error, its mass becomes
considerably reduced toward the direction to solve the level order puzzle
between the Roper resonance $N'(1440)$ and the negative-parity nucleon
$N^*(1535)$.
