A 16-year-old boy was arrested early Wednesday in Takasu, Hokkaido, after allegedly stabbing his older brother to death.
The incident took place late Tuesday night in the home of a 40-year-old single working mother whose three teenage sons often quarreled among themselves, police said.
They said the spat leading to the fatal fight broke out when the eldest brother, an 18-year-old high school student, became angry when the youngest, whose age was not provided, asked his mother to buy him a mobile phone.
When the older brother started punching the younger sibling, the middle brother took a 20-cm-long knife from the kitchen and stabbed him in the chest.
The victim was rushed to a hospital, where he was pronounced dead at about 1 a.m. Wednesday from loss of blood, hospital officials said.
The youth who was arrested reportedly told investigators he tried to help his younger brother and intended to kill the elder brother.
He was arrested after the mother called police.