A compact diagnostic system for monitoring Hard X-Ray (HXR) in the KSTAR tokamak has been developed in this work. Its development aims for an extension of the counting rate limit, and a good time resolution in the hard X-ray diagnostic. The all-in-one compact HXR diagnostic system is based on NaI(Tl) scintillation detector, because this scintillator provides reliable identification of the X-ray energy spectra with high efficiency. And in addition, the electronic equipment such as preamplifier, main amplifier, high voltage power supply, and fast analog-to-digital converter (ADC) with a digital signal processor (DSP) system was built-in on boards as compact modules in the system. In particular, a fast ADC based on a DSP, and an amplifier with a shaping time of 100 ns was adopted to achieve 1 ms time resolution and a higher counting rate up to 1 MHz. This diagnostic system is intended to provide information on dynamic mechanisms of the high-energy electrons arising from when runaway electrons interact with plasma-facing components in the KSTAR tokamak.