When there is currently no article at an upstream end of a trough of a downstream linear feeder, an upstream linear feeder is driven to operate until any article detected by an upstream article detector. In case no article is still detectable by the upstream article sensor when a predetermined period of time is passed after the upstream linear feeder starts to be driven, the upstream linear feeder is driven to operate by an increased vibration strength based on the assumption that an article(s) is being jammed or articles are being stuck together on an upstream side of the upstream linear feeder.