The jailing of a stepfather on Tuesday in a heart-rending case of child abuse has forced Japan to take a deep look at how it treats children.

The case comes amid signs that the plunge in the number of births is not abating, pointing to concerns about the treatment of children in Japanese society.

Yudai Funato, 34, pleaded guilty over his role in the death of his stepdaughter, Yua, five, in Tokyo's Meguro ward in March last year and was jailed for 13 years.

His former wife, Yuri, 27, who was last month jailed for being complicit by turning a blind eye to the abuse, has appealed against her eight-year sentence.

Funato said during his trial that his intent was to discipline Yua, on whom he had imposed his "image of an ideal child", and began assaulting her when he felt that his efforts were not working.

"I couldn't control my emotions," he said. "I believe it is completely my responsibility."

Yua was malnourished and had as many as 170 wounds all over her body when she died 18 days shy of her sixth birthday last year.

Her journal entries, begging for her parents' approval, included one that read: "Please, please, please forgive me. I will make sure I can do more things tomorrow than today without daddy and mummy having to tell me what to do."

A startling spate of child abuse cases has coincided with Japan's well-documented struggles to lift its birth rate.

The number of births this year is set to fall below 900,000 for the first time, government data shows.

Yet official data also shows that the number of cases reported to the childcare authorities has surged nearly 20 per cent to a record 159,850 in the year ended March - a climb that officials attribute to "greater social awareness".

Many incidents of abuse, observers note, occur in lower-income households that are facing immense stresses to begin with.

While Japan is registering record-low unemployment, as much as 40 per cent of the workforce is in dead-end "irregular" jobs with minimum salaries.

Funato had been unemployed, as was Yusuke Shindo, 32, who allegedly murdered his nine-year-old stepson Ryosuke last month.

Shindo is accused of strangling the boy in anger after Ryosuke, when told off for losing his cap, retorted: "Leave me alone. You are not my biological father."

While Shindo's alleged crime was judged as impulsive, Funato was found to be culpable of the near-daily abuse of his stepdaughter.

Sociologist Emi Kataoka of Tokyo's Komazawa University told The Straits Times that she sees a growing number of families facing social strains associated with tax increases and stagnant wages.

She said: "The stress faced by parents can easily lead to the abuse of children. And the reality is that the number of abuse cases is rising."