In this paper, an improved Schiffman phase shifter is proposed using slot-coupled lines. It consists of the typical coupled lines and a simple rectangular slot which is placed in the ground plane underneath the coupled lines. Phase responses of the slot-coupled Schiffman phase shifter are analyzed in consideration of the phase velocity of two independent transmission modes and the characteristic impedance of slot-coupled line. The $905^{circ}$ Schiffman phase shifters with high impedance ratio is realized at 2.4 GHz, which cannot be implemented with the conventional coupled lines. The measured results show that the bandwidth is around 74.2 % within the phase errors ${pm}5^{circ}$. The measured return loss is found to be better than 15.0 dB and insertion loss is less than 1.5 dB over the operating frequency band.