An image reading device, comprising: a light emitting portion; a stick-like light guiding member, of which end portion is disposed to face the light emitting portion; a holding member configured to hold the light emitting portion and the light guiding member; and a case member to which the holding member is attached, characterized in that: the holding member includes: a first positioned portion that is formed at one end part of the holding member in a main scanning direction; and a second positioned portion that is formed at another end part of the holding member in the main scanning direction, and wherein the case member includes: a first positioning portion which is formed integrally with the case member and into which the first positioned portion is inserted, the first positioning portion being configured to restrict the first positioned portion thus inserted from moving in a direction transverse to the main scanning direction; and a second positioning portion which is formed integrally with the case member and into which the second positioned portion is inserted, the second positioning portion being configured to restrict the second positioned portion thus inserted from moving in a direction transverse to the main scanning direction. a light emitting portion; a stick-like light guiding member, of which end portion is disposed to face the light emitting portion; a holding member configured to hold the light emitting portion and the light guiding member; and a case member to which the holding member is attached, characterized in that: the holding member includes: a first positioned portion that is formed at one end part of the holding member in a main scanning direction; and a second positioned portion that is formed at another end part of the holding member in the main scanning direction, and wherein the case member includes: a first positioning portion which is formed integrally with the case member and into which the first positioned portion is inserted, the first positioning portion being configured to restrict the first positioned portion thus inserted from moving in a direction transverse to the main scanning direction; and a second positioning portion which is formed integrally with the case member and into which the second positioned portion is inserted, the second positioning portion being configured to restrict the second positioned portion thus inserted from moving in a direction transverse to the main scanning direction. a first positioned portion that is formed at one end part of the holding member in a main scanning direction; and a second positioned portion that is formed at another end part of the holding member in the main scanning direction, and wherein the case member includes: a first positioning portion which is formed integrally with the case member and into which the first positioned portion is inserted, the first positioning portion being configured to restrict the first positioned portion thus inserted from moving in a direction transverse to the main scanning direction; and a second positioning portion which is formed integrally with the case member and into which the second positioned portion is inserted, the second positioning portion being configured to restrict the second positioned portion thus inserted from moving in a direction transverse to the main scanning direction. a first positioning portion which is formed integrally with the case member and into which the first positioned portion is inserted, the first positioning portion being configured to restrict the first positioned portion thus inserted from moving in a direction transverse to the main scanning direction; and a second positioning portion which is formed integrally with the case member and into which the second positioned portion is inserted, the second positioning portion being configured to restrict the second positioned portion thus inserted from moving in a direction transverse to the main scanning direction.