Stable the magnetic field intensity of the superconducting magnet in the driven mode operation and achieve the improvement of the image quality of the MRI image. Therefore, the magnetic locking sample 122 is disposed at a position to which the magnetic field generated by the superconducting coil 105 is applied, and is located at a position that does not physically interfere with the subject 101. The magnetic field lock section (123) irradiates the magnetic field lock sample 122 with a high frequency magnetic field, detects the NMR signal (B) generated by the magnetic field lock sample 122, adjusts the intensity of the static magnetic field according to the detection result, Constant static magnetic field. Preferably, the sample for magnetic field lock includes a nucleus species that generates a nuclear magnetic resonance signal (B) with a magnetic resonance frequency different from the magnetic resonance frequency of the nuclear magnetic resonance signal (A) detected by the measuring unit.ドリブンモード運転の超電導磁石の磁場強度を安定にし、MRI画像の画質向上を達成する。そのために、磁気ロック用試料122は、超電導コイル105が発生した磁場が印加される位置であって、被検体101と物理的に干渉しない位置に配置される。磁場ロック部(123)は、磁場ロック用試料122に高周波磁場を照射し、磁場ロック用試料122が発生したNMR信号(B)を検出し、検出結果に応じて静磁場の強度を調整し、静磁場を一定にする。好ましくは、磁場ロック用試料は、計測部が検出する核磁気共鳴信号(A)の磁気共鳴周波数とは異なる磁気共鳴周波数の核磁気共鳴信号(B)を発生する原子核種を含む。
