Balun device and differential phase shifter are provided. The balun device includes a first primary coil, a first secondary coil, a second primary coil and a second secondary coil, the first primary coil having a first terminal receiving a first differential signal, and a second terminal outputting a first in-phase component, the first secondary coil having a first terminal outputting a first component orthogonal to the first in-phase component, and a second terminal coupled to AC ground, the second primary coil having a first terminal receiving a second differential signal, and a second terminal outputting a second in-phase component; the second secondary coil having a first terminal outputting a second component orthogonal to the second in-phase component, and a second terminal coupled to AC ground; phase differences between the first and second differential signals, between the first and second in-phase components, between the first and second orthogonal component are 180°.