  The magnetic structure of the intermetallic compound EuGa$_4$ was
investigated using single-crystal neutron diffraction with the time-of-flight
(TOF) Laue technique on the new diffractometer SENJU at MLF of J-PARC. In spite
of high neutron absorption of Eu, a vast number of diffraction spots were
observed without isotope enrichment. The magnetic reflections were appeared at
the positions with the diffraction indices of $h+k+l{\neq}2n$ below 16 K,
indicating that the ordering vector is ${\bf q}=(0~0~0)$. Continuous evolution
of magnetic reflection intensity below $T_{\rm N}$ follows a squared Brillouin
function for $S$=7/2. By adopting a wavelength-dependent absorption collection,
the magnetic structure of EuGa$_4$ was revealed that a nearly full magnetic
moment of 6.4$~{\mu}_{\rm B}$ of Eu lies within the basal plane of the lattice.
The present study reveals a well-localized divalent Eu magnetism in EuGa$_4$
and demonstrates a high ability of SENJU to investigate materials with high
neutron absorption.
