  N103B is a Type Ia supernova remnant (SNR) in the Large Magellanic Cloud
(LMC). We carried out new $^{12}$CO($J$ = 3-2) and $^{12}$CO($J$ = 1-0)
observations using ASTE and ALMA. We have confirmed the existence of a giant
molecular cloud (GMC) at $V_\mathrm{LSR}$ $\sim$245 km s$^{-1}$ towards the
southeast of the SNR using ASTE $^{12}$CO($J$ = 3-2) data at an angular
resolution of $\sim$25$"$ ($\sim$6 pc in the LMC). Using the ALMA $^{12}$CO($J$
= 1-0) data, we have spatially resolved CO clouds along the southeastern edge
of the SNR with an angular resolution of $\sim$1.8$"$ ($\sim$0.4 pc in the
LMC). The molecular clouds show an expanding gas motion in the
position-velocity diagram with an expansion velocity of $\sim5$ km s$^{-1}$.
The spatial extent of the expanding shell is roughly similar to that of the
SNR. We also find tiny molecular clumps in the directions of optical nebula
knots. We present a possible scenario that N103B exploded in the wind-bubble
formed by the accretion winds from the progenitor system, and is now
interacting with the dense gas wall. This is consistent with a
single-degenerate scenario.
