The baby was at the baggage claim with its mother — a US national — and its Canadian father after a flight from London early Thursday morning in the eastern seaside city of Alicante, an airport spokeswoman told new agency AFP.
"The five-month old baby was on the belt. I don't know if it was placed on it directly, or in a cradle," she said.
"It was about two o'clock in the morning, on the belt for retrieving special baggage, where children's push chairs and large items come out," which was activated when the parents were not looking, the spokeswoman added.
"The airport medical service tried to resuscitate the baby but they could not do anything."
Police were "investigating the exact circumstances", a Civil Guard spokesman said.
"The mother left the baby on the conveyor belt, which can be activated by detecting weight or by an airport employee," the UK's Guardian newspaper reported the Civil Guard as saying.
"It seems that the baby carrier's weight activated the belt and the child was trapped in the rollers in the drop zone."
"It all appears to have been an accident, a case of carelessness."
The Red Cross is providing psychological assistance to the family, Spain's El País newspaper reported.
They were reported to be planning on a holiday in the popular seaside town of Denia.