The bus was carrying 47 workers who had just crossed the border into Thailand to work legally. Three other people on the bus were injured but the rest escaped unharmed, police said.
"The driver said he saw fire breaking out from the middle of the bus, which then spread quickly," Kritkanok Dan-udom, the chief of Maetor district police station, told Reuters.
Pictures in Thai media showed the burnt-out shell of the bus.
Thailand is estimated to have more than 3 million migrant workers, many of whom come from much poorer neighbour Myanmar. Work permit rules were tightened last year.
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