The most direct and strongest evidence for the presence of dark energy is provided by the measurement of galaxy distances using SNe Ia. This result is based on the assumption that the corrected brightness of SN Ia through the empirical standardization would not evolve with look-back time. Recent studies have shown, however, that the standardized brightness of SN Ia is correlated with host morphology, host mass, and local star formation rate (SFR), suggesting a possible correlation with stellar population property. To understand the origin of these correlations, we have continued our spectroscopic observations to cover most of the reported nearby early-type host galaxies. From high-quality (signal-to-noise ratio ∼175) spectra, we obtained the most direct and reliable estimates of population age and metallicity for these host galaxies.We find a significant correlation between SN luminosity (after the standardization) and stellar population age at a 99.5% confidence level. As such, this is the most direct and stringent test ever made for the luminosity evolution of SN Ia. Based on this result, we further show that the previously reported correlations with host morphology, host mass, and local SFR are most likely originated from the difference in population age.This indicates that the light-curve fitters used by the SNe Ia community are not quite capable of correcting for the population age effect, which would inevitably cause a serious systematic bias with look-back time. Notably, taken at face values, most of the Hubble residual used in the discovery of the dark energy appears to be affected by the luminosity evolution. ). According to the analysis by Childress et al. (2014), the difference in the mean age of SN progenitors between z = 0 and 1.0 is predicted to be ∼5.3 Gyr (see also Section 4 below). It is therefore possible that SNe at high redshift are fainter (after the standardization) than the value expected from the model without the dark energy, not because of the dark energy, but mostly because of the luminosity evolution. Careful investigation of this possible systematic bias with redshift is particularly important because the dimming of SNe is only ∼0.2 mag (∼20% in brightness) with respect to a cosmological model without dark energy (Ω M = 0.27, Ω Λ = 0.00; Hicken et al. 2009).Nevertheless, to test the effect of luminosity evolution, in the discovery papers, Riess et al. (1998), Schmidt et al. (1998), and Perlmutter et al. (1999 only used morphological classification of host galaxies in the local universe as a proxy for stellar population age. Because of apparently very small difference in the standardized brightness between SNe Ia in the early-type and late-type host galaxies, they concluded that the luminosity evolution is negligible in SN cosmology. Later analysis based on a larger sample by Hicken et al. (2009), however, found a systematic difference of ∼0.14 mag between the very early-and very late-type galaxies (see also Suzuki et al. 2012). Recent investigations of host galaxies further found a co...