diff --git a/executer/user/mcexec.1in b/executer/user/mcexec.1in
index 8b93bc63..f75cfd43 100644
--- a/executer/user/mcexec.1in
+++ b/executer/user/mcexec.1in
@@ -22,11 +22,15 @@ are described below.
 
 .TP
 .B	-n N
-Specify the number of MPI ranks run on the node.
-e.g.,
+Specify the number of MPI ranks run on the node when using MPI (except Fujitsu MPI).
+For example,
+.br
    $ mpirun -n 32 -ppn 4 mcexec -n 4 ./a.out
-In the above example, the ./a.out program runs on eight nodes each of
+.br
+the ./a.out program runs on eight nodes each of
 which has four processes.
+You must specify this option when using MPI (except Fujitsu MPI).
+Undefined behavior entails if not.
 
 .TP
 .B	-t N