diff --git a/actfw_core/v4l2/video.py b/actfw_core/v4l2/video.py
index 14ae90c..4edeaf6 100644
--- a/actfw_core/v4l2/video.py
+++ b/actfw_core/v4l2/video.py
@@ -10,6 +10,7 @@
 import os
 import select
 import time
+import warnings
 from ctypes import *
 from ctypes.util import find_library
 from typing import List
@@ -839,7 +840,11 @@ def __init__(self, device="/dev/video0", blocking=False):
             except OSError as e:
                 # retry 3 times when device is busy
                 if e.errno != errno.EBUSY or i == 2:
-                    raise RuntimeError("open {}: {}".format(self.device, errno.errorcode[e.errno]))
+                    raise RuntimeError(f"open {self.device}: {errno.errorcode[e.errno]}")
+            warnings.warn(
+                f"Retrying to open {self.device} after 1 second.",
+                RuntimeWarning,
+            )
             time.sleep(1)
 
         self.converter = _v4lconvert.create(self.device_fd)