An image reading apparatus comprising: a document sheet table having a top surface on which a document sheet is placed; a first carriage that has a lighting portion which applies light from a light source so as to diffuse the light toward the document sheet table in a primary scanning direction, and that can shuttle in a secondary scanning direction orthogonal to the primary scanning direction; a photoelectric converting portion arranged to receive, as image light, light obtained by the light applied by the lighting portion being reflected by an image surface of the document sheet, and that converts the received image light to an electrical signal; and a position detecting portion that detects an initial position in shuttling of the first carriage, the initial position being a position at which reading of the document sheet is initiated, wherein the light source is disposed at an end portion, in the primary scanning direction, of the lighting portion, the lighting portion includes: a light guide member on which the light from the light source is incident, and which guides the incident light in the primary scanning direction and applies the light to the document sheet table; and a heat dissipating member that dissipates heat generated in the light source, the position detecting portion includes: a light emitter that emits light; a light receiver that receives the light emitted from the light emitter; and a detection member that moves in the secondary scanning direction along with the shuttling of the first carriage, in order to detect the initial position of the first carriage, and the detection member is formed as a flat plate that is integrated with the heat dissipating member and extends in the secondary scanning direction. a document sheet table having a top surface on which a document sheet is placed; a first carriage that has a lighting portion which applies light from a light source so as to diffuse the light toward the document sheet table in a primary scanning direction, and that can shuttle in a secondary scanning direction orthogonal to the primary scanning direction; a photoelectric converting portion arranged to receive, as image light, light obtained by the light applied by the lighting portion being reflected by an image surface of the document sheet, and that converts the received image light to an electrical signal; and a position detecting portion that detects an initial position in shuttling of the first carriage, the initial position being a position at which reading of the document sheet is initiated, wherein the light source is disposed at an end portion, in the primary scanning direction, of the lighting portion, the lighting portion includes: a light guide member on which the light from the light source is incident, and which guides the incident light in the primary scanning direction and applies the light to the document sheet table; and a heat dissipating member that dissipates heat generated in the light source, the position detecting portion includes: a light emitter that emits light; a light receiver that receives the light emitted from the light emitter; and a detection member that moves in the secondary scanning direction along with the shuttling of the first carriage, in order to detect the initial position of the first carriage, and the detection member is formed as a flat plate that is integrated with the heat dissipating member and extends in the secondary scanning direction.