[en] The thorium target was irradiated by positrons from a ¹¹⁸Te-¹¹⁸Sb beta-plus source and electrons emitted from the target were measured by means of an electron spectrometer and a counter telescope consisting of two position sensitive counters. We have observed excess electrons located at 328.6 ± 0.3 keV in the spectrogram. The kinematic energy and the intrinsic FWHM of the electron line were deduced to be 333 ± 1 keV and 3.7 ± 0.3 keV, respectively and the cross section was estimated as 120 ± 40 mb. If we assume that the electrons result from the decay of an anomalous particle emitting an electron-positron pair with the same energy, its mass is to be 1688 ± 2 keV. (author)