We have proposed and investigated the tolerable performance of an optical serial-to-parallel converter with phase operation (OSPC) with lower power consumption than the conventional scheme for the purpose of achieving reduction of both the processing load and the processing latency of the high-speed packet switching systems in the data center. The target OSPC consists of Mach-Zehnder delay interferometers (MZDIs) and balanced photodetectors with silicon (Si) photonic technology. However, we have confirmed that the performance becomes degraded due to optical propagation losses of the delay lines in MZDIs. To solve this issue, unbalanced splitting ratio of a multi-mode interferometer (MMI)-type couplers in the MZDIs has been investigated, but we have found the necessity of improving the tolerance of the splitting ratio deviation. In this paper, we will analytically and experimentally demonstrate the improvement of the characteristics by using an OSPC with variable splitting ratio optical coupler (VSOC) consisting of a symmetric Mach-Zehnder interferometer (MZI). By observing the transmission spectra of the device with various applied voltage to the heater, controllability of peak-to-valley ratios of transmission spectra was observed. In addition, splitting ratio of the output symmetric MMI almost matches well with that of the optimized peak-to-valley ratios of transmission spectra by using short-optical pulse injection. We have confirmed the tolerable performance of the Si-photonic OSPC by using VSOC with the symmetric MZI.