A gear pump or a gear motor includes a casing, a helical drive gear, a helical driven gear, a drive-side space, and an idler-side space. The drive and driven gears mesh with each other in the casing to partition inside of the casing so as to include high and low pressure spaces. The drive-side and idler-side spaces are each configured to allow pressure therein to become higher than a pressure in the low-pressure space. The drive-side space faces an end portion of a drive shaft rotatably supporting the drive gear. The idler-side space faces an end portion of an idler shaft rotatably supporting the driven gear. The end portion of the drive shaft is pushed in a predetermined direction by working fluid supplied to the drive-side space. The end portion of the idler shaft is pushed in the predetermined direction by working fluid supplied to the idler-side space.