PROBLEM TO BE SOLVED: To provide a high-frequency signal-system circuit which enables continuous measurement of output waveforms of an RF signal in magnet resonance imaging in a CW operation method.SOLUTION: The high-frequency signal-system circuit in magnetic resonance imaging includes an RF output level detection circuit 15 constituted using a pair of peak-hold circuits, which detects a peak value of an RF signal output from a high-frequency amplification circuit 12 at a control timing of a control pulse (PH-SEL) and transmits the peak value to a processor 14.