Résumé : In an experiment performed at the 70 GeV IHEP accelerator the rate decay η'(958) → 3π0, which violates isosoin conservation, has been observed. The six final decay gamma rays are detected in the hodoscope spectrometer GAMS-2000. The branching ratio of this decay is BR (η' → 3π0) = (1.6 ± 0.4) × 10-3. An estimate of the ratio of the masses of u and d quarks is given. © 1984.