A photometric dispensing nozzle unit (10 1 -10 n) comprising: a nozzle (11 1 -11 n) having a distal end opening through which suction/discharge of gas is performed and to which a dispensing tip (211, 212) is attached; a light guide end portion (32 1 -32 n) which is provided in the nozzle (11 1 -11 n) and in which light is received or irradiated at a distal end of the nozzle (11 1 -11 n); a dispensing cylinder (12 1 -12 n) including a cylinder having a cavity (12c, 12r, 12q, 12p) therein, a plunger that is slidable in the cavity (12c, 12r, 12q, 12p), and a suction/discharge port through which suction/discharge of gas is performed; a suction/discharge flow path (71 1 -71 n) that passes through the nozzle (11 1 -11 n) and allows the suction/discharge port to communicate with the distal end opening of the nozzle (11 1 -11 n); and a light guide path (31 1 -31 n) optically connected to the light guide end portion (32 1 -32 n) through the nozzle (11 1 -11 n) without passing through the dispensing cylinder (12 1 -12 n), characterised in that the cavity (12c, 12r, 12q, 12p) of the dispensing cylinder (12 1 -12 n) has a first diameter region having a first diameter inner peripheral surface, and a second diameter region formed on the suction/discharge port side of the first diameter region and having a second diameter inner peripheral surface, wherein the first diameter is larger than the second diameter, the plunger has a thick shaft portion that is slidable in the first diameter region, and a thin shaft portion that protrudes from a distal end surface of the thick shaft portion in the longitudinal direction and is slidable in the second diameter region, wherein the cavity (12c, 12r, 12q, 12p) further comprises a play region in which the thick shaft portion is configured to move in the longitudinal direction, wherein the play region is not sealed with the thick shaft portion and has a maximum inner diameter larger than the first diameter and in which the thick shaft portion does not slide; wherein the play region is formed between the first diameter region and the second diameter region, and the suction/discharge port is formed so as to be located in a cavity ahead of the second diameter region. a nozzle (11 1 -11 n) having a distal end opening through which suction/discharge of gas is performed and to which a dispensing tip (211, 212) is attached; a light guide end portion (32 1 -32 n) which is provided in the nozzle (11 1 -11 n) and in which light is received or irradiated at a distal end of the nozzle (11 1 -11 n); a dispensing cylinder (12 1 -12 n) including a cylinder having a cavity (12c, 12r, 12q, 12p) therein, a plunger that is slidable in the cavity (12c, 12r, 12q, 12p), and a suction/discharge port through which suction/discharge of gas is performed; a suction/discharge flow path (71 1 -71 n) that passes through the nozzle (11 1 -11 n) and allows the suction/discharge port to communicate with the distal end opening of the nozzle (11 1 -11 n); and a light guide path (31 1 -31 n) optically connected to the light guide end portion (32 1 -32 n) through the nozzle (11 1 -11 n) without passing through the dispensing cylinder (12 1 -12 n), characterised in that the cavity (12c, 12r, 12q, 12p) of the dispensing cylinder (12 1 -12 n) has a first diameter region having a first diameter inner peripheral surface, and a second diameter region formed on the suction/discharge port side of the first diameter region and having a second diameter inner peripheral surface, wherein the first diameter is larger than the second diameter, the plunger has a thick shaft portion that is slidable in the first diameter region, and a thin shaft portion that protrudes from a distal end surface of the thick shaft portion in the longitudinal direction and is slidable in the second diameter region, wherein the cavity (12c, 12r, 12q, 12p) further comprises a play region in which the thick shaft portion is configured to move in the longitudinal direction, wherein the play region is not sealed with the thick shaft portion and has a maximum inner diameter larger than the first diameter and in which the thick shaft portion does not slide; wherein the play region is formed between the first diameter region and the second diameter region, and the suction/discharge port is formed so as to be located in a cavity ahead of the second diameter region.