  Stellar winds blown out from massive stars ($\gtrsim 10M_{\odot}$) contain
precious information on the progenitor itself, and in this context, the most
important elements are carbon (C), nitrogen (N), and oxygen (O), which are
produced by the CNO cycle in the H-burning layer. Although their X-ray
fluorescence lines are expected to be detected in swept-up shock-heated
circumstellar materials (CSMs) in supernova remnants (SNRs), particularly those
of C and N have been difficult to detect so far. Here, we present a
high-resolution spectroscopy of a young magnetar-hosting SNR RCW~103 with the
Reflection Grating Spectrometer (RGS) onboard XMM-Newton and report on the
detection of \ion{N}{7} Ly$\alpha$ (0.50~keV) line for the first time. By
comparing the obtained abundance ratio of N to O (N/O$=3.8 \pm{0.1}$) with
various stellar evolution models, we show that the progenitor of RCW~103 is
likely to have a low-mass (10--12~$M_{\odot}$) and medium-rotation velocities
($\lesssim 100~\rm{km~s^{-1}}$). The results also rule out the possibility of
dynamo effects in massive ($\geq35~M_{\odot}$) stars as a formation mechanism
of the associated magnetar 1E~161348$-$5055. Our method is useful for
estimating various progenitor parameters for future missions with
microcalorimeters such as XRISM and Athena.
