[en] Full text: This paper describes a diamagnetic double-loop method for the measurement of the stored plasma energy in the magnetized plasma. Two concentric diamagnetic loops enclose a plasma column in a stellarator. The merit of this method is a high ability to cancel out the rippling background magnetic field and the effect of eddy current to measure relatively small diamagnetic signals. The method is applied to the plasmas in the helical axis stellarator Heliotron J. The stored energy of 0.7 kJ is measured with the of ±0.02 kJ at the rippling magnetic field of 1 T in the rf discharges with heating power up to 400 kW at 53GHz. The double diamagnetic loop method improves the S/N ratio by six times that of the conventional single loop method. The magnetic diagnostics hardware, the method of analysis, the calibration, the experimental results, and the error estimation are presented. This paper also describes the time histories of the diamagnetic stored energy in the different magnetic configurations of the initial plasmas in the Heliotron J with R= 1.2 m, < a>≤0.2 m, B≤1.5 T, ι/2π=0.3-0.8, L=1/M=4 . The stored energy in hydrogen plasmas is measured as a function of magnetic field strength (0.61 T< < B>axis <1.44 T). The value of Wp ∼0.7 kJ, which corresponds to <β>∼0.2%, is obtained by the second harmonic ECH at < B>axis 0.95 T with the input power 400 kW, ι/2π=0.55, < a>=0.18 m and < R>axis=1.20 m. The values of Wp ∼0.8 kJ and <β>∼0.1% are obtained at < B>axis ∼ 1.44 T by the off-axis ECH. The wave forms of the diamagnetic signals will be presented for the preliminary magnetic configuration scan with the vertical field coils and the toroidal coils, controlling the plasma position, the rotational transform, the minor radius, the ripples, etc. Copyright (2002) Australian National University- Research School of Physical Sciences and Engineering