A valve device includes a housing integrally or separately having a flow path formation part where at least one flow path hole through which a fluid passes is formed. The valve device includes: a drive part that outputs a rotational force; and a rotation part that rotates about a predetermined axis by the rotational force output from the drive part. The rotation part includes a shaft and a rotor increasing or decreasing an opening degree of the flow path hole with rotation of the shaft. The rotor has a sliding surface that slides while facing an opening surface of the flow path formation part where the flow path hole is opened. At least a part of the rotation part is rotatably held by the housing.