We demonstrated the monolithic 3D (M3D) integration of InGaAs photodetectors (PDs) on silicon-on-insulator metal-oxide-semiconductor field-effect transistors (SOI-MOSFETs) by the sequential process of InGaAs PDs on pre-fabricated SOI-MOSFETs. InGaAs PDs and SOI MOSFETs showed their original performances after integration and sequential process thanks to the low-temperature process. In addition, the integrated devices successfully performed the fundamental readout circuit operation such as direct injection and source follower per detector (SFD) by connecting transistors (Trs) and PDs. By illuminating 1550 nm laser on InGaAs PDs, the different behaviors of output voltages were clearly obtained according to the Tr's on/off state. From these results, we believe that this monolithic 3D integration method could be a feasible approach toward high-resolution multicolor imaging systems.