  The degeneration of the hyperelliptic sigma function is studied. We use the
Sato Grassmannian for this purpose. A simple decomposition of a rational
function gives a decomposition of Pl\"ucker coordinates of a frame of the Sato
Grassmannian. It then gives a decomposition of the tau function corresponding
to the degeneration of a hyperelliptic curve of genus $g$ in terms of the tau
functions corresponding to a hyperelliptic curve of genus $g-1$. Since the tau
functions are described by sigma functions, we get the corresponding formula
for the degenerate hyperelliptic sigma function.
