To achieve a long life and prevent mixture of dust to a sample air when the air is switched by moving back and forth a nozzle by means of a movement mechanism, switching and connecting selectively the nozzle to the other suction opening when the nozzle is separated from one suction opening.

For instance, when a sample air SA is completely measured at a suction opening h8, a drive control circuit 46 controls a movement mechanism 54 to retreat (move rightward) a rotary arm 51 by an axial tube 52 and separate a nozzle N from the suction opening h8 by a gap (g). Then, after a motor 551 rotates the arm 51 by 20° via a cam disk 552 to make the nozzle N meet a suction opening h9, the axial tube 52 is moved forward to connect the nozzle N with the suction opening h9. The sucked air SA is supplied to a dust detector 42. During this time, a connection position code generation circuit 47 sends a position (number) of the suction opening h9 detected from a detection signal of an angle detector to a data-processing control apparatus 44. The control apparatus 44 receives a count value from a counter 43 as a detection value from the dust detector 42 and stores the value as data of the suction opening h9.