diff --git a/NumLib/ODESolver/TimeDiscretization.cpp b/NumLib/ODESolver/TimeDiscretization.cpp
index 8eda8c3c8be5d73ac3a7c6296a1859fd4daa4d73..9a7d414f873fe2546bdb75ca789b70ddf79a4ee9 100644
--- a/NumLib/ODESolver/TimeDiscretization.cpp
+++ b/NumLib/ODESolver/TimeDiscretization.cpp
@@ -15,8 +15,9 @@
 
 namespace NumLib
 {
+
 double computeRelativeNorm(GlobalVector const& x,
-                           GlobalVector const& x_prev,
+                           GlobalVector const& y,
                            MathLib::VecNormType norm_type)
 {
     if (norm_type == MathLib::VecNormType::INVALID)
@@ -28,8 +29,8 @@ double computeRelativeNorm(GlobalVector const& x,
     GlobalVector dx;
     MathLib::LinAlg::copy(x, dx);  // copy x to dx.
 
-    // dx = x - x_prev --> x - dx --> dx
-    MathLib::LinAlg::axpy(dx, -1.0, x_prev);
+    // dx = x - y --> x - dx --> dx
+    MathLib::LinAlg::axpy(dx, -1.0, y);
     const double norm_dx = MathLib::LinAlg::norm(dx, norm_type);
 
     const double norm_x = MathLib::LinAlg::norm(x, norm_type);
diff --git a/NumLib/ODESolver/TimeDiscretization.h b/NumLib/ODESolver/TimeDiscretization.h
index d86e368a4dc22f03c4f8f45871961aecc969509b..4e1363e9c5c1631ae2bfdb57c9b2b93e0c0648bc 100644
--- a/NumLib/ODESolver/TimeDiscretization.h
+++ b/NumLib/ODESolver/TimeDiscretization.h
@@ -19,16 +19,14 @@
 namespace NumLib
 {
 /**
- * Compute and return the relative norm of solutions between two
- * successive time steps by \f$ \|u^{n+1}-u^{n}\|/\|u^{n+1}\| \f$.
+ * Compute the relative norm between two vectors by \f$ \|x-y\|/\|x\| \f$.
  *
- * \param x         The current solution
- * \param x_prev    The previous solution
+ * \param x Vector x
+ * \param y Vector y
  * \param norm_type The norm type of global vector
- * \return          \f$ \|u^{n+1}-u^{n}\|/\|u^{n+1}\| \f$.
+ * \return \f$ \|x-y\|/\|x\| \f$.
  */
-double computeRelativeNorm(GlobalVector const& x,
-                           GlobalVector const& x_prev,
+double computeRelativeNorm(GlobalVector const& x, GlobalVector const& y,
                            MathLib::VecNormType norm_type);
 
 //! \addtogroup ODESolver