A device (100) for measuring the amount of water in a subject's body includes a main body part (110) formed in a linear shape and an insertion part (120) extending in a curved shape from one end of the main body part (100). The insertion part (120) includes a tip side insertion part (120-2) having a tip surface (122) to which a sensor unit (121) is fixed and a base end side insertion part (120-1), wherein the sensor unit (121) is brought into contact with a body surface of a subject through a slide mechanism. A protective member (651) liquid-tightly covers the slide mechanism between the tip side insertion part (120-2) and a base end side insertion part (120-1). The protective member (651) smoothly connects the outer peripheral surface of the tip side insertion part (120-2) and the outer peripheral surface of the base end side insertion part (120-1).