Improvements in efficiency of a fuel-cell-powered vehicle have been studied using water electrolysis as the energy storage mechanism. Three methods are proposed for this purpose: 1. The reformer and fuel cell are divided into two or more units, and the maximum output of each unit is set to be small, which reduces the partial load operation, 2. all the fuel cell units are operated above the low efficiency partial load condition and excess electricity is supplied to another fuel cell unit to generate hydrogen and oxygen by water electrolysis, and these gases are compressed and stored, and arbitrary fuel cell units are supplied and they generate electricity, 3. deceleration periods perform water electrolysis of the fuel cell units using the electric power generated by the drive motor, and both gases are compressed and they store in each cylinder. The LA4 cycle (EPA urban dynamometer schedule) was introduced for the vehicle operation. The energy saving effects of the abovementioned methods were studied and were shown to increase the energy efficiency by 1.23 to 1.72 times compared to that for the conventional method.