A water treatment apparatus (10; 10A) that subjects water to be treated to ultraviolet treatment using ultraviolet light, the water treatment apparatus (10; 10A) comprising: a flow channel through which water to be treated flows; an ultraviolet irradiation portion that is located on an opposite side of the flow channel from an inlet side, and irradiates the water to be treated flowing in the flow channel with ultraviolet light via a window portion toward the inlet side, the window portion being formed by a transparent member; a treated water flow channel through which treated water obtained as a result of ultraviolet-treating the water to be treated flows; and an outlet through which the treated water flows out of the treated water flow channel, wherein the flow channel includes a small-diameter portion, a diameter increase portion (7; 7A) and a large-diameter portion larger in inner diameter than the small-diameter portion, in this order from the inlet side along the flow direction of the water to be treated, the treated water flow channel is formed using an outer pipe located on the outside of the large-diameter portion and constituting a double pipe structure with the large-diameter portion, an edge of the large-diameter portion on the opposite side from the inlet side in the flow direction of the water to be treated terminates while leaving a gap with the window portion, an end of the outer pipe on one side in the axial direction is connected in a watertight manner to an edge of a pipe forming the large-diameter portion of the flow channel on the inlet side, an end of the outer pipe on the other side in the axial direction is connected in a watertight manner to the window portion, and the diameter increase portion (7; 7A) is provided with an ultrasonic oscillator used to clean the window portion; and a sensor that detects the dirt state of the window portion wherein the diameter increase portion has a reverse tapered shape that gradually increases in inner diameter in a flow direction of the water to be treated. a flow channel through which water to be treated flows; an ultraviolet irradiation portion that is located on an opposite side of the flow channel from an inlet side, and irradiates the water to be treated flowing in the flow channel with ultraviolet light via a window portion toward the inlet side, the window portion being formed by a transparent member; a treated water flow channel through which treated water obtained as a result of ultraviolet-treating the water to be treated flows; and an outlet through which the treated water flows out of the treated water flow channel, wherein the flow channel includes a small-diameter portion, a diameter increase portion (7; 7A) and a large-diameter portion larger in inner diameter than the small-diameter portion, in this order from the inlet side along the flow direction of the water to be treated, the treated water flow channel is formed using an outer pipe located on the outside of the large-diameter portion and constituting a double pipe structure with the large-diameter portion, an edge of the large-diameter portion on the opposite side from the inlet side in the flow direction of the water to be treated terminates while leaving a gap with the window portion, an end of the outer pipe on one side in the axial direction is connected in a watertight manner to an edge of a pipe forming the large-diameter portion of the flow channel on the inlet side, an end of the outer pipe on the other side in the axial direction is connected in a watertight manner to the window portion, and the diameter increase portion (7; 7A) is provided with an ultrasonic oscillator used to clean the window portion; and a sensor that detects the dirt state of the window portion wherein the diameter increase portion has a reverse tapered shape that gradually increases in inner diameter in a flow direction of the water to be treated.