An absorbance detector for a chromatograph includes a light source, a flow cell that receives light generated by the light source, a photodetector that detects light transmitted through the flow cell, a light shield member that shields at least part of the light generated from the light source to the flow cell, a motor that rotates the light shield member to change the light shield member between a light-shielding state where the light shield member shields at least part of the light generated from the light source to the flow cell and a non-light-shielding state where the light shield member does not shield the light generated from the light source to the flow cell, and a reference position detector that rotates the motor and also acquires a change in amount of light received by the photodetector to detect a reference position of the light shield member in a rotation direction of the light shield member based on the acquired change in the amount of the received light.