A device for optical switches and logic gates is proposed in two-dimensional photonic crystals based on self-collimated beams. The main structure of the device is a line-defect-induced 3 dB splitter. Investigations show that the phase response of the self-collimated beams in the photonic crystals resembles that of the plane waves propagated in a homogeneous medium. Operating principle, as revealed by both theoretical calculation and finite-difference time-domain simulation, is based on an interference of reflected and transmitted beams of incident self-collimated beams. This device is potentially applicable for photonic integrated circuits.