A heat transfer device includes a bag and a working fluid. The bag includes a first sheet and a second sheet with edges that are sealed together. The working fluid is enclosed in the bag. The working fluid changes a phase thereof between gas and liquid. The bag includes a vaporizing portion in which the liquid-phase working fluid is vaporized and a condensing portion in which the gas-phase working fluid is condensed. The bag includes a two-phase flow channel in which liquid-gas two-phase slug flow including the liquid-phase working fluid and the gas-phase working fluid occurs from the vaporizing portion to the condensing portion. The two-phase flow channel is provided in an internal space of the bag.