We discuss γ-ray emissions from the outer gap accelerators of middle-aged pulsars for part of the series of our studies. A two-dimensional electrodynamic model is used to solve the distribution of accelerating electric fields with electron and positron pair creation and radiation processes in the magnetic meridional plane. We compute the curvature radiation and the synchrotron radiation by solving the evolution of the Lorentz factor and the pitch angle. The calculated spectra are compared with observed phase-averaged spectra. We also use a three-dimensional geometrical model to discuss the pulse profiles. We argue that the outer gap of middle-aged pulsars occupies the whole region between the last-open field lines and the critical magnetic field lines, which are perpendicular to the rotational axis at the light cylinder. We assume that there is no outer gap accelerator inside the light cylinder between the rotational axis and the critical magnetic field lines. For the Geminga pulsar, we demonstrate that the outward curvature radiation dominates in the spectrum above 10 MeV, while the inward synchrotron radiation dominates below 10 MeV. We find that the computed spectrum is consistent with the observations in X-ray through γ-ray bands. With the pulse morphology of the γ-ray emissions, we argue that the inclination angle and the viewing angle for the Geminga pulsar are α∼ 50° and ξ∼ 90°, respectively. We also apply our method to another four middle-aged radio pulsars, whose spin-down power and distance from the Earth expect the possibility of detection of γ-ray emissions from those pulsars by Fermi telescope. Applying the inclination angle and the viewing angle inferred from radio polarization characteristics, the visibility of the γ-ray emissions from the outer gap is discussed. We show that γ-ray emissions from PSRs B0355+54, B1449−64 and B1929+10 will probably be detectable by Fermi telescope. For PSR B0740−28, on the other hand, γ-ray emissions will not be detected because the γ-ray beam from the outer gap will be oriented in a different direction from the viewing angle.