For uncertain or unknown input systems, regular robust observers are usually constructed under some restrictive assumptions. Interval observer has more universality because it relaxes many restrictive assumptions and only needs the known boundary of the disturbance. Interval observer designing problems for both discrete and continuous descriptor systems with unknown inputs are considered. Discrete and continuous descriptor systems are respectively transformed into a form in which the coefficient matrices of interval observer can be obtained much more easily. Then the same interval observer design framework is developed. Simulation results show the effectiveness and correctness of the proposed method.