Given the parallelism between the physical properties of Ce- and Yb-based magnets and heavy fermions due to the electron-hole symmetry, it has been rather odd that the transition temperature of the Yb-based compounds is normally very small, as low as similar to 1 K or even lower, whereas Ce counterparts may often have the transition temperature well exceeding 10 K. Here, we report our experimental discovery of the transition temperature reaching 20 K in a Yb-based compound at ambient pressure. The Mn substitution at the Al site in an intermediate valence state of alpha-YbAlB4 not only induces antiferromagnetic transition at a record high temperature of 20 K but also transforms the heavy-fermion liquid state in alpha-YbAlB4 into a highly resistive metallic state proximate to a Kondo insulator.