Valve device, cooling water control device, and cooling water circuit

The disclosure relates to a valve device, a cooling water control device, and a cooling water circuit to highly accurately changing a flow rate in a low opening range. The valve device (1) includes a rotational shaft (2), and a valve body (4) internally forming a space (10) and rotatable about the rotational shaft (2), the valve body (4) having an outer circumferential surface (28) where a first communication hole (36) and a bottomed groove (38) are formed, the first communication hole (36) communicating with the space (10), the bottomed groove (38) extending from the first communication hole (36) toward one side in a rotational direction of the rotational shaft (2), and the groove (38) including a first section in which at least one of a depth (D) of the groove (38) and a width (W) of the groove (38) increases toward the first communication hole (36). 本公开涉及阀装置、冷却水控制装置以及冷却水回路，其目的在于使低开度区域中的流量高精度地变化。阀装置(1)具备：旋转轴(2)；以及阀芯(4)，其在内部形成空间(10)，并能够以旋转轴(2)为中心旋转，在阀芯(4)的外周面(28)上形成有与空间(10)连通的第一连通孔(36)以及从第一连通孔(36)向旋转轴(2)的旋转方