Ultraviolet light fluid treatment device

An ultraviolet light fluid treatment device for irradiating a fluid flowing in a flow path tube with ultraviolet light, the ultraviolet light fluid treatment device comprising:

• a substrate;
• a light source disposed on the substrate and configured to emit the ultraviolet light;
• a light transmissive member disposed between the light source and the flow path tube and configured to transmit the ultraviolet light from the light source;
• and
• a support member fixed to the substrate and configured to hold an adsorption member that includes an adsorbent material adsorbing moisture,
• wherein the support member includes:
• a through hole inside of which the light source is disposed in a plan view as seen in a direction normal to a main surface of the light transmissive member;
• a recessed portion which is located outside the through hole in the plan view and is open towards the light transmissive member;
• and
• a wall portion partitioning the through hole and the recessed portion,
• wherein an inner surface of the wall portion defining the through hole is a reflective surface, and the adsorption member is disposed within the recessed portion such that the wall portion is located between the light source and the adsorption member.