  We establish the exact triangle in Seiberg-Witten-Floer theory relating the
monopoloe homologies of any two closed 3-manifolds which are obtained from each
other by $\pm 1$-surgery. We also show that the sum of the modified version of
the Seiberg-Witten invariants for any closed rational homology 3-sphere $Y$
over all $Spin^c$ structures equals to $\frac 12 |H_1(Y, \Z)| \lambda (Y)$
where $\lambda (Y)$ is the Casson-Walker invariant.
