To improve luminance uniformity in a light irradiation region in an ultraviolet irradiation device having a light source element in which an oblong lamp is inserted into an overcoat tube.

A light source part 10 includes light source elements 20 in which oblong lamps 25 are inserted into oblong overcoat tubes 24 having light permeability, which are arranged in parallel at equal intervals. Cooling air produced by a cooling fan 42 is introduced from one end side through a first air guide duct 43A to cooling air flow paths 24A of the light source elements 20, flows from a right side to a left side while cooling the lamps 25 and is discharged from the other end side. The discharged cooling air is introduced from a second air guide duct 43B to a heat exchanger 43 and cooled. Cylindrical light guides 51 having reflection surfaces inside are arranged between the light source part 10 and a work stage 60. Reflection light amount decreasing means 55 decreasing an amount of reflecting light is provided on a part of a reflection surface of the cylindrical light guides 51 provided on a side to which the cooling air is discharged.