A gene-mutated animal such as a mouse which comprises a mutant prsenilin-1 gene comprising a DNA having a sequence encoding a mutant presenilin-1 protein in which an amino acid is substituted with a different amino acid in an amino acid sequence of a presenilin-1 protein; for example, a mutant presenilin protein in which isoleucine at position 213 is substituted with an amino acid other than isoleucine, e.g., threonine, in a mouse presenilin-1 protein. The animal is useful as an animal model which has pathological conditions closer to a human patient with Alzheimer's disease.