We consider a new action of a two-dimensional field theory interacting with gravitational field. The action is interpreted as the area of a surface imbedded into four-dimensional Mincowski target space. In addition to reparametrization invariance the new action has one extra infinite-dimensional local symmetry with a clear geometrical meaning. The special gauge choice, which includes the gauge condition of tracelessness of the energy-momentum tensor, leads to an effective free scalar field theory. The problem of anomalies in quantum theory and possible connection with matrix quantum mechanics are also discussed.