A fuel cell stack, comprising stacked cells each including: a sheet-shaped power generation portion; two separators that hold the power generation portion in a thickness direction; a gas passage defining plate disposed between at least one of the two separators and the power generation portion, the gas passage defining plate including a gas passage portion through which reactant gas flows; and a frame member disposed around the gas passage defining plate, the frame member including a supply port through which the reactant gas is supplied to the gas passage portion and a discharge port through which the reactant gas is discharged from the gas passage portion, wherein a flow direction of the reactant gas flowing from the supply port toward the discharge port is referred to as a flow direction, a direction in which the power generation portion opposes the gas passage portion is referred to as an opposing direction, a direction that is orthogonal to the flow direction and the opposing direction is referred to as an orthogonal direction, the gas passage portion includes opposing portions extended in the flow direction and arranged in parallel in the orthogonal direction, the opposing portions opposing the power generation portion, a main passage is defined between each of the opposing portions and the power generation portion, the gas passage portion includes connection passages each connecting the main passages to each other and being spaced apart from each other in the flow direction, the gas passage portion includes a first passage portion and a second passage portion, the first passage portion being adjacent to the supply port in the flow direction, and the second passage portion being adjacent to the first passage portion in the orthogonal direction, and the main passages of the second passage portion each have a larger cross-sectional flow area than the main passages of the first passage portion. a sheet-shaped power generation portion; two separators that hold the power generation portion in a thickness direction; a gas passage defining plate disposed between at least one of the two separators and the power generation portion, the gas passage defining plate including a gas passage portion through which reactant gas flows; and a frame member disposed around the gas passage defining plate, the frame member including a supply port through which the reactant gas is supplied to the gas passage portion and a discharge port through which the reactant gas is discharged from the gas passage portion, wherein a flow direction of the reactant gas flowing from the supply port toward the discharge port is referred to as a flow direction, a direction in which the power generation portion opposes the gas passage portion is referred to as an opposing direction, a direction that is orthogonal to the flow direction and the opposing direction is referred to as an orthogonal direction, the gas passage portion includes opposing portions extended in the flow direction and arranged in parallel in the orthogonal direction, the opposing portions opposing the power generation portion, a main passage is defined between each of the opposing portions and the power generation portion, the gas passage portion includes connection passages each connecting the main passages to each other and being spaced apart from each other in the flow direction, the gas passage portion includes a first passage portion and a second passage portion, the first passage portion being adjacent to the supply port in the flow direction, and the second passage portion being adjacent to the first passage portion in the orthogonal direction, and the main passages of the second passage portion each have a larger cross-sectional flow area than the main passages of the first passage portion.