The present invention provides an antibody or peptide which immunologically reacts with human VEGF receptor Flt-1 and cells in which human VEGF receptor Flt-1 is expressed on the cell surface and an antibody or peptide which inhibits binding of human VEGF to human VEGF receptor Flt-1. It also provides a means for the diagnosis or treatment of diseases in which their morbid states progress by abnormal angiogenesis, such as proliferation or metastasis of solid tumors, arthritis in rheumatoid arthritis, diabetic retinopathy, retinopathy of prematurity, psoriasis, and the like.