To enable a sample tube rack to fit respective sample tubes which have different lengths and diameters, without adversely a function of placing or taking out of the sample tubes into or from a sample tube rack.

A sample rack 10, having a front shell 12 and a back shell 14 jointed mutually so as to form a clamshell structure, is equipped with tube receiving chambers 1. Each tube receiving chamber 1 is equipped with a tube engaging cradle arranged, so as to be engaged with the surface part of a sample tube inserted into each tube receiving chamber 1. Each of parts 54, 78, 82 of the tube engaging cradle is formed on the front shell 12 and the back shell 14. The structure of the tube engaging cradle is completed by mutual jointing of each shell. Each tube receiving chamber 1 is equipped with an energizing spring leg 192, which energizes a sample tube toward the tube engaging cradle. The sample rack 10 is equipped with spring fixing parts 48, 70, supporting energizing springs 20, 22 which are equipped with the energizing spring leg 192.