diff --git a/practice/synchronized1/CommonCalculate.java b/practice/synchronized1/CommonCalculate.java
new file mode 100644
index 000000000..dbe5e041f
--- /dev/null
+++ b/practice/synchronized1/CommonCalculate.java
@@ -0,0 +1,28 @@
+package synchronized1;
+
+public class CommonCalculate {
+  private int amount;
+  
+  public CommonCalculate() {
+    this.amount = 0;
+  }
+
+  Object lock = new Object();
+
+  public void plus(int value) {
+    // ...
+    // 10만 줄....
+    // ...
+    synchronized(lock) {
+      this.amount += value;
+    }
+  }
+  
+  public void minus(int value) {
+    this.amount -= value;
+  }
+
+  public int getAmount() {
+    return this.amount;
+  }
+}
diff --git a/practice/synchronized1/ModifyAmountThread.java b/practice/synchronized1/ModifyAmountThread.java
new file mode 100644
index 000000000..0252b2f40
--- /dev/null
+++ b/practice/synchronized1/ModifyAmountThread.java
@@ -0,0 +1,22 @@
+package synchronized1;
+
+public class ModifyAmountThread extends Thread {
+    private CommonCalculate calc;
+    private boolean addFlag;
+
+    public ModifyAmountThread(CommonCalculate calc, boolean addFlag) {
+        this.calc = calc;
+        this.addFlag = addFlag;
+    }
+
+    @Override
+    public void run() {
+        for (int i = 0; i < 10000; i++) {
+            if (addFlag) {
+                calc.plus(1);
+            } else {
+                calc.minus(1);
+            }
+        }
+    }
+}
diff --git a/practice/synchronized1/RunSync.java b/practice/synchronized1/RunSync.java
new file mode 100644
index 000000000..f3258d637
--- /dev/null
+++ b/practice/synchronized1/RunSync.java
@@ -0,0 +1,30 @@
+package synchronized1;
+
+public class RunSync {
+  
+  public static void main(String[] args) {
+    RunSync runSync = new RunSync();
+    for (int i = 0; i < 5; i++) {
+      runSync.runCommonCalculate();
+    }
+  }
+
+  public void runCommonCalculate() {
+    CommonCalculate calc = new CommonCalculate();
+
+    ModifyAmountThread thread1 = new ModifyAmountThread(calc, true);
+    ModifyAmountThread thread2 = new ModifyAmountThread(calc, true);
+
+    thread1.start();
+    thread2.start();
+
+    try {
+      thread1.join(); // 쓰레드가 종료될 때까지 기다리는 메소드
+      thread2.join();
+
+      System.out.println("calc : "+calc.getAmount());
+    } catch (InterruptedException e) {
+      e.printStackTrace();
+    }
+  }
+}