When the father awoke, he found the boy unconscious in the drum of the machine and called emergency services for help on Jan. 27. The child was pronounced dead in a hospital.
Police say the boy may have climbed into the washer out of curiosity and that the lid slammed shut, trapping him inside. It cannot be opened from the inside.
Although the machine was not on, nor filled with water, the interior is airtight. A doctor who examined the body said there were no external injuries, but signs of suffocation.
Police were to conduct an autopsy, possibly on Jan. 29.
According to Osaka prefectural police, the boy's 32-year-old father found his son unconscious, then called for an ambulance around 3 p.m.
The front-loader is on the first floor of the house. It stands 1 meter tall and measures 70 centimeters by 60 cm.
It was fitted with a child lock mechanism, which was not activated.
On that afternoon, the boy's 32-year-old mother had gone out. After having lunch, the father took a nap in a second-floor room with his son. When he awoke, the boy was not around.
Many fatal accidents involving front-loading washing machine have been reported since 2008 in South Korea, the United States and elsewhere.
A 7-year-old boy in Tokyo's Ome city was found suffocated in a front loader in June 2015.
Japan's Consumer Affairs Agency warned of the dangers of front-loaders in e-mailed newsletters to the public in 2014 and 2015.
It encouraged households to always keep the machine's lid closed and keep it fastened with a large rubber band.
In their instruction manuals, manufacturers also warn of the dangers and produce warning stickers to be placed on the washers.
The Japan Electrical Manufacturers’ Association recommends using a child safety lock and keeping the door locked when the washer is off, not just while in use.
Many models are now fitted with the devices.
After the 2015 tragedy in Ome, Japanese makers heightened their safety measures. A model was released that allowed the lid of the drum to be opened from the inside.
“Children naturally do unexpected things while playing,” said Hiromi Nakatsuji, a nurse with Miyako Anshin Kodomo Kan, a public service run by the Kyoto city government pushing for greater public awareness to prevent accidents at home involving children.
“Children aged 4 or 5 years old can understand warnings given by parents," she said. "Parents must educate them about the dangers of climbing into the washing machine or the washbasin.”
Nakatsuji said she had "heard of families that do not allow their children to enter the washroom by themselves.”