A fuel cell unit structure includes: power generation cells; separators; a flow passage portion formed between the separators and including flow passages configured to supply gas to the power generation cells; gas flow-in ports configured to allow the gas to flow into the flow passage portion; gas flow-out ports configured to allow the gas to flow out from the flow passage portion; and an adjustment portion configured to adjust an amount of the gas flowing through the flow passages. The adjustment portion includes a first auxiliary flow passage provided between the power generation cells arranged to be opposed to each other on a same plane with a gas flow-in port of the gas flow-in ports being located on an extended line of an extending direction of the first auxiliary flow passage.