diff --git a/to-lower-case/sonshn.java b/to-lower-case/sonshn.java
new file mode 100644
index 0000000000..030d6a92a0
--- /dev/null
+++ b/to-lower-case/sonshn.java
@@ -0,0 +1,21 @@
+/**
+ * 시간 복잡도: O(n)
+ * 공간 복잡도: O(n)
+ */
+class Solution {
+    public String toLowerCase(String s) {
+        StringBuilder sb = new StringBuilder();
+
+        for (int i = 0; i < s.length(); i++) {
+            char c = s.charAt(i);
+
+            if (c >= 'A' && c <= 'Z') {
+                c = (char) (c + 32);
+            }
+
+            sb.append(c);
+        }
+
+        return sb.toString();
+    }
+}