A beam position monitor with high precision and resolution is required to control the beam trajectory for matching to the injection orbit and acceleration in a heavy-ion synchrotron. It will be also used for measuring the beta function, tune, and chromaticity. Since the bunch length at heavy ion synchrotron is relatively long, a few meters, a boxlike device with plates of typically 20cm length is used to enhance the signal strength and to get a precise linear dependence with respect to the beam displacement. Especially, the linear-cut beam position monitor is adopted to satisfy the position resolution of 100µm and accuracy of 200µm for a nominal beam intensity in the KHIMA synchrotron of ∼7×108 particles for the carbon beams and ∼2×1010 for the proton beams. In this paper, we show the electromagnetic design of the electrode and surroundings to satisfy the resolution of 100µm, the criteria for mechanical aspect to satisfy the position accuracy of 200µm, the measurement results by using wire test-bench, design and measurement of a high input impedance pre-amplifier, and the beam-test results with long (∼1.6μs) electron beam in Pohang accelerator laboratory (PAL).