A model of a solar coronal transient in the course of its formation is constructed. The main idea is that the formation of a coronal transient occurs at a quasi-stationary state of the solar atmosphere as a result of the interaction of weak coronal currents with a slowly growing magnetic field originating below the future transient area. The explanation of forerunner formation which takes place much before the appearance of the transient itself is one of the verifications of the model. The application of the model to the transient event of Jan. 10, 1974, demonstrates the reasonable values of model parameters for reproducing the observed columnar density distribution. 18 references.