In this article, for any a in the MS-algebra L, the author considers the congruence of the form defined by (x,y)∈θ_a \Leftrightarrow x∧a^\circ =y∧a^\circ and x∨a^{\circ\circ}=y∨a^{\circ\circ} . A characterization of subdirectly irreducible MS-algebras is given and it is also proved that the union of two principal congruences having this form is a principal congruence if and only if L∈K₂∨K₃.