Here She Is, the Saint in Japan.

She was supposed to have been summoned as a saintess, but was thrown into a country called Japan. Rei Natsukawa, a.k.a. Alexandra Ollier, hides her magic power as she works for a small company in modern Japan. She says that she'll be happy with a peaceful and uneventful life, but several small obstacles stand in the way of her lifestyle. This is the ordinary, yet not-so-ordinary story of a powerful mage.

(Source: Akita Shoten, translated)