According to one embodiment, there is provided with sheet processing device including an ultrasonic wave generator, a detecting array and a judging unit. The ultrasonic wave generator irradiates ultrasonic waves to a sheet being conveyed, from one side of the sheet. The detecting array is arranged in the other side of the sheet and detects sound pressure of the ultrasonic waves passing through the sheet. The judging unit judges a state of the sheet by comparing a predetermined standard value with the sound pressure.