In this paper, a quasi-deadbeat property is introduced and a quasi-deadbeat minimax filter (DMMF) is proposed for continuous-time state space signal models. Linearity, quasi-deadbeat property, FIR structure, and independence of the initial state information will be required in advance, in addition to a performance index of the worst case gain between the disturbance and the current estimation error. The proposed DMMF is obtained by directly minimizing a performance index with the quasi-deadbeat constraint. The proposed DMMF is represented first in a standard FIR form and then in an iterative form. The DMMF will be shown to be used also for the IIR structure.