MoS2, a representative 2D atomically thin semiconductor, has a sizable band gap leading to intensive research efforts to investigate its unique optical properties and realize a novel optoelectronic device based on MoS2. However, limited optical absorption in extremely thin MoS2 layers is an obstacle for high-efficiency light absorbing devices. In this work, we investigated how reflection and transmission phase-shift at the highly absorbing MoS2 interface could affect the absorption spectra of the MoS2 monolayers on SiO2/Si substrates (SiO2 thickness: 40 ~ 130 nm). Such interface-phase-shift gave rise to interference in MoS2 layer, although the layer thickness was only 0.7 nm, much smaller than the wavelength of the visible light. We compared measured and calculated optical reflection spectra, which showed that aforementioned interface-interference enhanced optical absorption in MoS2 monolayers. Raman intensity of MoS2 monolayers largely varied depending on the SiO2 thickness, which could be well explained by the calculated absorption in MoS2 layers. In addition, the interface-interference enabled omnidirectional absorption enhancement. This work showed that proper choice of the SiO2 thickness could provide us a simple and useful means to improve optical absorption in MoS2 monolayers.