The paper provides the first demonstration of the efficiency of applying the magneto-optic method to studies of the spatial and temporal magnetic-field relaxation in an YBa2Cu3O7 film strip after the transport current is switched on. It is shown that the evolution of magnetic flux distribution is adequately described in terms of a modified Bean model with time-dependent critical current. At a time 50 ms after the current is switched on, the critical current of the samples studied decreases by 15%. This proves the significance of thermally activated magnetic flux motion (creep)in the regime investigated. The magnetic vortex pinning energy has been estimated as U0 = 20 kT.