diff --git a/src/Domain/Cart.php b/src/Domain/Cart.php
index 5522428..66d4749 100644
--- a/src/Domain/Cart.php
+++ b/src/Domain/Cart.php
@@ -12,7 +12,12 @@ class Cart
 
     public function add(string $productId, Price $unitPrice, int $amount = 1)
     {
-        $this->items[] = new Item($productId, $unitPrice, $amount);
+        try {
+            $item = $this->find($productId);
+            $item->add($amount);
+        } catch (ProductNotInCartException $e) {
+            $this->items[] = new Item($productId, $unitPrice, $amount);
+        }
     }
 
     public function remove(string $productId)
@@ -35,4 +40,17 @@ public function calculate(): CartDetail
 
         return new CartDetail($detailItems, new Price($totalPrice));
     }
+
+    /**
+     * @throws ProductNotInCartException
+     */
+    private function find(string $productId): Item
+    {
+        foreach ($this->items as $item) {
+            if ($item->getProductId() === $productId) {
+                return $item;
+            }
+        }
+        throw new ProductNotInCartException();
+    }
 }
diff --git a/src/Domain/Item.php b/src/Domain/Item.php
index 26beaa9..5391dc4 100644
--- a/src/Domain/Item.php
+++ b/src/Domain/Item.php
@@ -33,6 +33,11 @@ public function toDetail(): DetailItem
         return new DetailItem($this->productId, $this->unitPrice, $this->amount);
     }
 
+    public function getProductId(): string
+    {
+        return $this->productId;
+    }
+
     public function add(int $amount)
     {
         $this->checkAmount($amount);
diff --git a/src/Domain/ProductNotInCartException.php b/src/Domain/ProductNotInCartException.php
new file mode 100644
index 0000000..8be6385
--- /dev/null
+++ b/src/Domain/ProductNotInCartException.php
@@ -0,0 +1,10 @@
+<?php
+
+namespace Simara\Cart\Domain;
+
+use DomainException;
+
+class ProductNotInCartException extends DomainException
+{
+
+}