Three more people, including a 13-year-old boy, were wounded in Sunday's attack during the final of a local league tournament in the town of Paracho, the state prosecutor's office said.
The unknown assailants fled the scene and nobody has been detained so far.
The federal government has deployed thousands of troops to Michoacan in a bid to bring peace to a state plagued by drug-related violence.
Last month, the government legalised vigilante militias that formed last year to combat the Knights Templar drug cartel.