Abstract: Provided are a therapeutic or prophylactic agent for retinopathy of prematurity (ROP) that is suited to the pathogenic mechanism of ROP and a method of testing for ROP. The therapeutic or prophylactic agent for ROP uses at least one substance from the group consisting of inhibitors against tryptase derived from mast cells and/or mast cell stabilizers as an active ingredient. The testing method for ROP includes detecting a marker substance that can be released by degranulation of mast cells in a biological sample originating from a patient and determining the presence or absence of ROP on the basis of the detected amount of the marker.