According to an aspect of the present invention, there is provided an oil pressure sensor attaching structure in which a plurality of guide protrusion portions are disposed with a gap in a circumferential direction and surround an oil passage opening portion. The guide protrusion portions have an arc-shaped wall portion and a protrusion portion. An accommodation portion which has an accommodation opening portion opening upward is provided on an inside of the plurality of guide protrusion portions in a radial direction. The sensor has a sensor case which has a columnar portion, a plurality of flange portions, and an annular portion that surrounds the columnar portion. The flange portion is disposed between an upper surface of an oil passage body and the protrusion portions in the vertical direction at a first position in the circumferential direction. The annular portion has a first portion and a second portion.