The properties of terahertz (THz) wave modulation in organic-deposited metallic slit arrays fabricated on a Si substrate were investigated. The organic thin layer (fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester, also called PCBM) acts as an active layer controlled by the incident light and thus modulates the transmission of incident THz waves. This is due to the photo-excited electron transfer from a silicon substrate to the PCBM layer, which makes the organic layer metallic. The metallic slit arrays show the plasmonic property such as wavelength-dependent transmission which depends on the structural arrangements of slits. When the polarization of incident THz waves is parallel to the direction of metallic slits, the cut-off frequency appears clearly whereas, when the polarization perpendicular, the transmission is largely enhanced by the plasmonic property. The hybrid structures of organic-metallic slits-deposited on a silicon substrate enables active control of spoof-surface plasmon-mediated transmission of THz waves. This finding may be useful for realizing multi-functional THz devices and for designing spectrally controllable THz wave modulators for THz sensing or imaging systems.