Disclosed is a dual-mode ramjet engine including: a combustor through which air flowing in from the front passes; a sliding injector configured to perform a sliding operation from an upper surface of the combustor along a height direction perpendicular to an air flow direction; and a cavity-type flame holder formed behind the sliding injector on the upper surface of the combustor and having a cavity that is concavely recessed, wherein the sliding injector includes: a plurality of fuel injection holes formed on both sides and configured to inject fuel; a fuel injection passage passing through the inside along the height direction to be opened downward, and configured to inject fuel; and a first sliding drive unit configured to drive the sliding injector to perform the sliding operation.