The necessity to develop a magnetometer for variations research in the midfields magnetic strength with a relative error of 10–6 is justified. A modernization design of magnetometer to nuclear magnetic resonance with using to maser with flowing liquid is presented. The block diagram of nuclear magnetic resonance magnetometer is given. The core principle of its working is described. The results of experimental investigations of various variations to magnetic fields are presented. Further directions of modernization of the design of the magnetometer are determined.