PURPOSE: To enhance measurement accuracy, by providing the inside of a cell with a slidable body consisting of an element and vessels contg. correction solns. fixed to the element, removing matters attached to the inside of the cell, and always executing corrections.

CONSTITUTION: A slidable body 24 slidably inserted into the inside of a cell consists of an element 24a made of a magnetic material, such as a magnet, and vessels 24b, 24c contain a zero point correcting soln. 24d and a span correcting soln. 24e, respectively. When the body 24 is located in the right side of the cell 22, a sample (A) passing through a path 20 comes into the cell 22, and UV rays pass through the sample (A). When a power supply controller 25 is controlled to change over currents flowing to coils 23, 23', the body 24 shifts to the left side of the cell 22, a sample (B) flowing through a path 21 enters the cell 22, and the UV rays pass through this sample (B). Sliding the body 24 to the right and left sides permits alternate measurements of the samples (A), (B), and removal of matters attached to the inside of the cell 22, and hence, both of zero point and span corrections.