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We present results of the $^{12}$CO (1--0) mosaic observations of the nearbybarred-spiral galaxy M83 obtained with the Atacama LargeMillimeter/submillimeter Array (ALMA). The total flux is recovered by combiningthe ALMA data with single-dish data obtained using the Nobeyama 45-m telescope.The combined map covers a $\sim$13 kpc$^{2}$ field that includes the galacticcenter, eastern bar, and spiral arm with a resolution of \timeform{2''.03}$\times$ \timeform{1''.1} ($\sim$45 pc $\times$ $\sim$25 pc). With a resolutioncomparable to typical sizes of giant molecular clouds (GMCs), the COdistribution in the bar and arm is resolved into many clumpy peaks that formridge-like structures. Remarkably, in the eastern arm, the CO peaks form twoarc-shaped ridges that run along the arm and exhibit a distinct difference inthe activity of star formation: the one on the leading side has numerous HIIregions associated with it, whereas the other one on the trailing side has onlya few. To see whether GMCs form stars with uniform star formation efficiency(SFE) per free-fall time (SFEff), GMCs are identified from the data cube andthen cross-matched with the catalog of HII regions to estimate the starformation rate for each of them. 179 GMCs with a median mass of 1.6 $\times$10$^{6}$ $M_{\odot}$ are identified. The mass-weighted average SFEff of theGMCs is $\sim$9.4 $\times$ 10$^{-3}$, which is in agreement with models ofturbulence regulated star formation. Meanwhile, we find that SFEff is notuniversal within the mapped region. In particular, one of the arm ridges showsa high SFEff with a mass-weighted value of $\sim$2.7 $\times$ 10$^{-2}$, whichis higher by more than a factor of 5 compared to the inter-arm regions. Thislarge regional variation in SFEff favors the recent interpretation that GMCs donot form stars at a constant rate within their lifetime.

ALMA $^{12}$CO (J=1--0) imaging of the nearby galaxy M83: Variations in the efficiency of star formation in giant molecular clouds