  Thermodynamical properties of an interacting system of scalar bosons at
finite temperatures are studied within the framework of a field-theoretical
model containing the attractive and repulsive self-interaction terms.
Self-consistency relations between the effective mass and thermodynamic
functions are derived in the mean-field approximation. We show that for a
sufficiently strong attractive interaction a first-order phase transition
develops in the system via the formation of a scalar condensate. An interesting
prediction of this model is that the condensed phase appears within a finite
temperature interval and is characterized by a constant scalar density of Bose
particles.
