  We investigate low-energy properties of the alternating spin chain model
composed of spin $s_1$ and $s_2$ with a singlet ground state. After examining
the spin-wave spectrum in detail, we map low-energy spin excitations to the
O(3) non-linear sigma model in order to take into account quantum fluctuations.
Analyzing the topological term in the resulting sigma model, we discuss how the
massless or massive excitations are developed, especially according to the
topological nature of the alternating spin system.
