  The notion of strong external difference family (SEDF) in a finite abelian
group $(G,+)$ is raised by M. B. Paterson and D. R. Stinson [5] in 2016 and
motivated by its application in communication theory to construct $R$-optimal
regular algebraic manipulation detection code. A series of
$(n,m,k,\lambda)$-SEDF's have been constructed in [5, 4, 2, 1] with $m=2$. In
this note we present an example of (243, 11, 22, 20)-SEDF in finite field
$\mathbb{F}_q$ $(q=3^5=243).$ This is an answer for the following problem
raised in [5] and continuously asked in [4, 2, 1]: if there exists an
$(n,m,k,\lambda)$-SEDF for $m\geq 5$.
