A beam combining device and a projector having such a beam combining device are provided. The projector includes a first color light source, a second color light source, a third color light source, a beam combining device, a light valve and a projection unit. The first color light source, the second color light source and the third color light source respectively emit first, second and third color beams. The beam combining device includes a polarization state converting module, a color separation unit and a polarization beam splitter. The optical path lengths of the first color beam emitted from the first color light source to the polarization beam splitter, the second color beam emitted from the second color light source to the polarization beam splitter, and the third color beam emitted from the third color light source to the polarization beam splitter are equal.