Hi Gisela. Some photos of small mammals are difficult because the key characteristics are not in photos, such as the notched incisors and mousy smell of a house mouse. And scale. Juveniles are more difficult still, which is what this might be. At first sight this looks like either a house mouse or Pseudomys novaehollandiae then I suspected it may be a juvenile rat, either a Bush Rat or a Black Rat. The tails of Pseudomys are lighter below, and this does not seem to have that, but it does have large ears. My guess is House Mouse. I will ask some other people to help.