The present invention provides a non-human model mammal of Alzheimer's disease (AD) containing chimeric amyloid precursor protein (APP) gene capable of producing human amyloid .beta. peptide (A.beta.) or a living part thereof, characterized in that A.beta.42/A.beta.40 ratio at 8-weeks-old is about 7-fold or more (about 140-fold or more in homozygote) higher compared to that of a corresponding wild-type mammal. Moreover, the present invention provides the mammal or a living part thereof, further characterized in that the level of APP expression is not significantly different compared to the corresponding wild-type mammal, and a screening method for a prophylactic and/or therapeutic drug for AD, a biomarker in biological fluids and molecular imaging of amyloid deposition or other pathological changes in the brain for an early diagnosis of AD using the mammal or a living part thereof.