  We study T-duality for open strings on tori $\T^d$. The general boundary
conditions for the open strings are constructed, and it is shown that T-duality
group, which preserves the mass spectrum of closed strings, preserves also the
mass spectrum of the open strings. The open strings are transformed to those
with different boundary conditions by T-duality. We also discuss the T-duality
for D-brane mass spectrum, and show that the D-branes and the open strings with
both ends on them are transformed together consistently.
