An anime film adaptation of Mizuki Tsujimura's fantasy novel Kagami no Kojou (English edition title: Lonely Castle in the Mirror) is now in the works for a release in the winter of 2022. Shochiku will work on distribution. Further details such as production company and staff are currently unknown.

The novel was published by Poplar Publishing in Japan in May 2017, and its total print run has surpassed 1.2 million copies. The novel's manga adaptation illustrated by Tomo Taketomi was serialized in Shueisha's Ultra Jump from June 2019 to February 2022, and its final chapter was published in the magazine's latest March 2022 issue published on February 19. 

The anime film's official website introduces its story as below:

One day, a mirror in Kokoro's room suddenly began to glow in front of her eyes, as she had lost her place at school and was confined to her room. She walked through the shining mirror to find a mysterious castle-like building. There were seven people in a similar situation to Kokoro. It is said that a secret "key" is hidden in the castle, and whoever finds it will have their wish come true. Why were these seven people gathered together? and where is the key? When everything is revealed, you will be surprised and moved.

Source: Shochiku Movie official Twitter, the anime film's official website 

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