A fluid sterilization apparatus comprising: a housing that forms a flow passage through which a fluid subject to sterilization passes; a light source that is provided outside the housing, where the light source has a semiconductor light-emitting element that emits ultraviolet light; a light receiving unit provided outside the housing that receives a portion of the ultraviolet light emitted from the light source and detects the amount of ultraviolet light that has been received; and a control unit that controls the output of the semiconductor light-emitting element based on information regarding the amount of ultraviolet light acquired from the light receiving unit, wherein the housing has an incidence portion which transmits ultraviolet light from outside towards the inside of the housing, a cylindrical sterilization chamber that has a reflection portion that reflects the ultraviolet light at the inner surface thereof, and an inlet port for the fluid and an outlet port for the fluid, which are provided in a direction intersecting with the longitudinal direction of the sterilization chamber, wherein the light source is arranged outside at least one of both sends of the sterilization chamber in the longitudinal direction, wherein the incidence portion is provided between the light source and the sterilization chamber, wherein the reflection portion is formed of a material that transmits a portion of ultraviolet light, and wherein the light receiving unit is provided at a position where the light receiving unit can receive the portion of ultraviolet light that has been transmitted through the reflection portion; wherein the light receiving unit is arranged in a region opposite to the incidence portion across the reflection portion, and the reflection portion is formed of a material whose diffuse reflectance for ultraviolet light having a wavelength of 285 nm is in a range of 78.5 to 95.4 percent and whose diffuse transmittance for ultraviolet light having a wavelength of 285 nm is in a range of 0.6 to 13.1 percent. a light source that is provided outside the housing, where the light source has a semiconductor light-emitting element that emits ultraviolet light; a light receiving unit provided outside the housing that receives a portion of the ultraviolet light emitted from the light source and detects the amount of ultraviolet light that has been received; and a control unit that controls the output of the semiconductor light-emitting element based on information regarding the amount of ultraviolet light acquired from the light receiving unit, wherein the housing has an incidence portion which transmits ultraviolet light from outside towards the inside of the housing, a cylindrical sterilization chamber that has a reflection portion that reflects the ultraviolet light at the inner surface thereof, and an inlet port for the fluid and an outlet port for the fluid, which are provided in a direction intersecting with the longitudinal direction of the sterilization chamber, wherein the light source is arranged outside at least one of both sends of the sterilization chamber in the longitudinal direction, wherein the incidence portion is provided between the light source and the sterilization chamber, wherein the reflection portion is formed of a material that transmits a portion of ultraviolet light, and wherein the light receiving unit is provided at a position where the light receiving unit can receive the portion of ultraviolet light that has been transmitted through the reflection portion; wherein the light receiving unit is arranged in a region opposite to the incidence portion across the reflection portion, and the reflection portion is formed of a material whose diffuse reflectance for ultraviolet light having a wavelength of 285 nm is in a range of 78.5 to 95.4 percent and whose diffuse transmittance for ultraviolet light having a wavelength of 285 nm is in a range of 0.6 to 13.1 percent.