if the result in step 93 is affirmative the processing proceeds to step 95 the process in step 94 which is entitled 34 cd on cw 34 is shown in detail in fig9 f as shown in fig9 f the arm 100 is turned counterclockwise until the optical position detector c or d is turned on steps 137 138 and 139 thereby positions of the arm 100 and the wafer 30 are changed as shown in fig1 g 11 g and 12 g at step 95 which is entitled 34 cd off cw 34 and is shown in detail in fig9 g the arm 100 is turned clockwise until both the optical position detectors c and d are turned off steps 140 141 and 142 subsequently a process in step 102 is carried out the process in step 102 is entitled 34 arm sl 34 and is shown in detail in fig9 h at step 102 as shown in fig1 h 11 h and 12 h the arm 100 is contracted until all the optical position detectors a b and c are turned off steps 143 144 and 145 thereby positions of the wafer 30 and the arm 10 are changed as shown in fig1 i 11 i and 12 i it is noted that the center of the wafer 30 becomes placed on the x axis then the arm 100 is driven downwards and the sucking operation is disabled so that the wafer 30 is placed on the centering stage 40 step 103 then the arm 100 is moved to the center position of the centering unit 12 step 104 and then wafer 30 is held by the suction hole 24 by activating the sucking operation step 105 further the aforementioned process shown in fig9 e is carried.