Three observables of early-type galaxies - size ($r_{e}$), surface brightness ($I_{e}$), and velocity dispersion ($sigma_{0}$) - form a tight planar correlation known as the fundamental plane (FP), which has provided great insights into the galaxy formation and the evolution processes. However, the FP has been found to be tilted against the simple virial expectation, prompting debates on its origin. In order to investigate the contribution of systematic stellar population variation to the FP tilt, we study here the FP relations of early-type galaxies in mid-infrared (MIR) which may represent the stellar mass well. We examined the wavelength dependence of the FP coefficients, $a$ and $b$ in $log r_{e}= alogsigma_{0} + blog< I >_{e} + c$, using a sample of 56 early-type galaxies for which visible (V-band), near-infrared (K-band), and MIR (Spitzer IRAC, 3.6--8.0$mu$m) data are available. We find that the coefficient $a$ increases as a function of wavelength as $da/dlambda=0.11pm0.04mu m^{-1}$, while the coefficient $b$ reaches the closest to -1 at 3.6--5.8$mu$m. When applied to the visible FP coefficients derived from a larger sample of nearby early-type galaxies, we get the FP relation with $(a,b) simeq $(1.6--1.8,-0.9) at 3.6$mu$m. Our result suggests that the stellar population effect can explain more than half of the FP tilt, closing the gap between the virial expectation and the optical FP. The reduction in the FP tilt is reflected in the dynamical mass-to-light ratio, $M_{dyn}/L$, dependence on $L$ which decreases toward 3.6--5.8$mu$m, suggesting that the MIR light better represents mass than the shorter wavelengths.
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