  We calculate a grand partition function of the attractive Bose gas in the
infinite space within some approximations. Using the idea of the Yang-Lee
zeros, it is proved that the gas-liquid condensation occurs before the
conventional condition of the Bose-Einstein condensation is satisfied. Further,
it is pointed out that Bosons with a zero momentum play a role of a trigger to
this gas-liquid condensation. We discuss its implication to the trapped atomic
gas.
