Two-phase closed thermosyphons are applied to the gas-to-gas heat exchangers, the prevention of icing of roads, the melting of snow, the cooling of electronic devices and so on. However, when the heat fluxes supplied to the thermosyphon are small, the temperatures and pressure in the thermosyphon oscillate because the geysering of working fluid occurs due to the intermittent boiling. The present study attempts to restrain the oscillation by supplying bubble nuclei. A tubular sinter was inserted into the working fluid at the bottom of the thermosyphon to supply bubble nuclei. The experimental results show that the amplitudes of oscillating temperatures and pressure in the thermosyphon were decreased by inserting the tubular sinter.