Emi Mamiya, 29, was arrested on suspicion of attempted murder after apparently slashing her husband's face with the knife, police said.
Mamiya was reportedly incensed by the smell left behind after her 34-year-old partner emerged from the stall.
Her anger was further fuelled when he tried to help their 3-year-old son use the toilet "without washing his own hands first", she said, Jiji Press and other local media reported.
"The toilet stinks so much that the child cannot even use it," Jiji reported her as saying.
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