The present invention can solve such a problem that a conventional tissue piece treating apparatus has difficulty of reduction in size and restriction on installation and stopping subsequent treatment operations when the amount of chemical supplied to a processing chamber is insufficient. The tissue piece treating apparatus comprises: new chemical tanks 15, 17 being integrally attached to both sidewall faces of a main body of the apparatus respectively, having larger capacities than a plurality of chemical tanks 14, 16 in which chemicals with different concentrations are stored for performing immersion treatment of a tissue piece accommodated in a processing chamber 12, having inner wall faces respectively reinforced by reinforcing members, and storing unused new chemicals of the same types as the chemicals stored in the chemical tanks 14, 16; pipes being extended from the new chemical tanks 15, 17 and connected to a selection valve 30 so as to supply the new chemicals stored in the new chemical tanks 15, 17 to the processing chamber 12; and a control section 38 controlling the selection valve 30, an open/close valve 32 and a pump 34 so as to supply the new chemical of an amount corresponding to an insufficiency of the amount of the chemical supplied from the new chemical tanks 15,17 to the processing chamber 12 when a detection sensor 36 detects that the amount of the chemical supplied to the processing chamber 12 is insufficient.