Hydrocarbon fuel oil with paraffins as the main constituent for use in fuel cell systems is provided having (1) paraffins of less than 13 carbons constitute not less than 60% by mass; (2) isoparaffins of 15 or more carbons constitute from 0.01 to 9% by mass; and (3) normal paraffins of 13 or more carbons constitute not more than 2% by mass. Also, in addition to the aforementioned compositions (1) to (3), it is preferable if isoparaffins of 17 or more carbons constitute not more than 3% by mass.