Tenkuu Shinpan Arrive

Senya is just an ordinary boy, who wanted to become an astronaut as a kid. One day and suddenly, he's transported into another world after seeing on the rooftop of his school something like a "tower" going across the sky. There, he witnesses a ruined world, where skyscrapers are connected by suspension bridges. Senya, perplexed, manages to see a girl standing on the roof of another building, as well as a masked man holding a bloody weapon in his hands...

(Source: Kodansha, translated)