In order to clarify the unstable vibration of a flexible disk coupled with a head and suspension system, the effects of constant friction force and other parameters of the head and suspension system are analyzed using a disk-head coupling model. In this model, it is assumed that the transverse and pitching motion of upper and lower head sliders interacts with a disk through transverse and pitching contact springs. The constant friction force destabilizes forward eigenmodes of the spinning disk, and this instability is largely suppressed by head mass and transverse suspension stiffness. However, it is found that this suppression effect remains small near the transverse natural frequency of the head and suspension system only when the upper and lower transverse contact spring stiffnesses are different. It is also found that the pitching vibration of the head amplifies the instability near the pitching natural frequency, while suppressing the instability in other frequency regions. Further, if the transverse and pitching natural frequencies of the head and suspension system coincide, strong instability is induced at this frequency.