  The concept of work is studied in quantum thermostatistics of a system
surrounded by an environment and driven by an external force. It is found that
there emerges the gauge theoretical structure in a nonequilibrium process, the
field of which is referred to as the work gauge field. The thermodynamic work
as the flux of the work gauge field is considered for a cyclic process in the
space of the external-force parameters. As an example, the system of a spin-1/2
interacting with an external magnetic field is analyzed. This geometric effect
may be observed, for example, in an NMR experiment and can be applied to the
problem of cooling/heating of a small system.
