A fusing device has a heater, a fusing rotary member heated thereby, a pressing rotary member in contact with the fusing rotary member to form a fusing nip, an adjustment mechanism adjusting a fusing pressure to adjust the nip width of the fusing nip, a temperature sensing member, and a control portion. By the control portion, the fusing pressure is set at a first set value during fusing warm-up operation for raising temperature of the fusing rotary member, is set at a third set value when a first sheet of paper after completion of fusing warm-up operation passes through the fusing nip, and is changed to a second set value before sheets of paper for a preset number of pages pass through the fusing nip after completion of fusing warm-up operation. Of the set values, the third is greatest, the second is second greatest, and the first is smallest.