This technical note considers the delay margin problem of single-input single-output finite-dimensional linear time-invariant (FDLTI) plant with two or more real unstable poles by using LTI control. This research is the extension and further results of the previous result on plant with one real unstable pole. Explicit upper bound of the achievable delay margin is provided based on a frequency domain approach, and the bound can be transformed to the maximum value of a function of two variables.