Two-dimensional (2D) molybdenum disulfide (MoS2) field-effect transistors (FETs) have been extensively studied, but most of FETs with gate insulator have displayed negative threshold voltage values, which indicate the presence of interfacial traps whether they are shallow or deep in energy level. Despite such interface trap issues, reports on trap density in MoS2 are quite a few. Here, we probed the top-gate MoS2 FETs with two- (2L), three- (3L) and four-layer (4L) MoS2/dielectric interfaces to quantify any deep-level interface trap densities by photo-excited charge collection spectroscopy (PECCS), reporting a result that deep-level trap density over 1012 cm-2 may exist in the interface and bulk MoS2 near the interface. Transfer curve hysteresis and PECCS measurements show that mobile shallow traps and deep traps are not much different in density order each other. We conclude that our PECCS analysis distinguishably provides valuable information on deep level interface trap density in 2D-based FETs.