A suction unit comprises reciprocating change-over suction device 1 including tubular body 1a having with passage 2 and two branch passages 2a and 2b bifurcated from passage 2 connected to a suction apparatus. Cylinder 1b is movable in body 1a and has ventilation passages 4 and 5 and actuation plates 8 and 9 fixed to ends of cylinder 1b. Chambers 6 and 7 are connected to the end portions of body 1a with closing members closing openings of the chambers. Elastic containers 10 and 11 are contained in chambers 6 and 7, openings of these containers being fixed to inner surfaces of closing members 14 and 15. Closed valves 12 and 13 are on chambers 6 and 7 and suction tubes 18a and 18b are connected to inflow passages 16a and 16b of members 14 and 15 and, at another end, are connected to common suction tube 18. Discharging tubes 19a and 19b are connected to outflow passages 17a and 17b of members 14 and 15 and, at another end, to storage container 20.