A DLP optical system is provided. The DLP optical system includes a lamp unit, an optical engine unit, a DMD, a prism, a projection lens, and a light-blocking part. The lamp unit provides light, and the optical engine unit is optically connected with the lamp unit, has optical parts for converting light incident from the lamp unit into an image signal therein, and includes a primary lens for transmitting the image signal. The DMD reflects the image signal transmitted from the primary lens, and the prism is formed closely to the DMD to adjust a path of light. The projection lens projects an on-beam of light from the prism that contributes to forming of an image onto a screen. The light-blocking part is provided on an incident side of the projection lens to transmit only the on-beam and shields light of a flat state region located between an on-beam and an off-beam.