A screw device is provided which applies preload with a single nut and can easily detect preload. The screw device of includes: a screw shaft having an outer helical groove; a nut fitted on the screw shaft, the nut having an inner helical groove, and a return path to which a passage formed between the outer groove and the inner groove is connected; a plurality of rolling elements placed between the passage and the return path; and a strain sensor attached to an outer surface of the nut. At least one hole is provided between an attachment surface, to which the strain sensor is attached, of the nut and/or a vicinity thereof and an inner surface of the nut.