A foreign matter detection unit detects an entry of an unexpected foreign matter that is other than the substrates by light transmitting optical sensors 7 having light projecting portions 7a and light receiving portions 7b at opening sections 3a, 3b formed in cover members 2 through which the substrates pass. By executing an emergency stop processing, when the entry of the foreign matter is detected at one of the opening sections, to stop the electronic component mounting apparatus that is adjacent to the opening section at which the entry of the foreign matter is detected, safety of the machine operators is properly ensured in check and confirmation works conducted at the time of machine troubles etc even in an electronic component assembling line in which the small-sized thin unit apparatuses are combined.