With the aim of reducing noise due to a digital signal in an A-D conversion unit, the following is implemented: an input unit is provided with four A-D converters and a gray code conversion unit. An analog NMR signal corresponding to each channel is inputted to each A-D converter, and the gray code conversion unit converts a digital signal represented in binary code, outputted from the A-D converters, into a digital signal represented in gray code. A gray code-converted digital signal is transferred to an output unit through a digital signal bus. The gray code digital signal is returned to a binary digital signal by a gray code inverse conversion unit, and then the output unit transmits a binary signal, outputted from the gray code inverse conversion unit, to a signal processing unit.