A flow rate adjusting valve includes: a casing including an inlet, an outlet, and a channel connecting the inlet and the outlet to each other; a valve port disposed in the channel; and a valve body that is disposed upstream of the valve port and moves forward and backward relative to the valve port to adjust an opening degree of an upstream opening of the valve port. The valve port includes a narrowing portion whose opening diameter is smaller than that of the upstream opening, and an enlarging portion continuous to a downstream side of the narrowing portion and having an opening diameter of the narrowing portion. When the upstream opening is at a predetermined opening degree, an area of the valve port through which a fluid passes is at minimum at the narrowing portion.