The energy conversion process as well as the plasma heating mechanism in the starting‐up phase of driven reconnection is investigated in detail using two‐dimensional magnetohydrodynamic (MHD) simulation. It is revealed that the compressional heating effect is predominant in this phase rather than the Ohmic and the viscous heating. The predominance of the compressional heating means that the plasma flow energy is considerably converted to the thermal energy. The strong compression takes place in the process during which the fast shock is generated in the downstream region of the reconnection point. The plasma heating due to nonsteady reconnection is briefly discussed as a promising mechanism for the anomalous ion heating, which is observed in reversed‐field pinch (RFP) experiments.