--- spack-src/setup.py.old	2020-10-28 10:48:02.800927663 +0900
+++ spack-src/setup.py	2020-10-28 10:49:44.755977753 +0900
@@ -24,12 +24,20 @@
 
 install_requires = ['addict', 'numpy', 'pyyaml']
 
-# If first not installed install second package
-CHOOSE_INSTALL_REQUIRES = [('opencv-python-headless>=3', 'opencv-python>=3')]
-
-for main, secondary in CHOOSE_INSTALL_REQUIRES:
-    install_requires.append(choose_requirement(main, secondary))
 
+try:
+    # OpenCV installed via conda.
+    import cv2  # NOQA: F401
+    major, minor, *rest = cv2.__version__.split('.')
+    if int(major) < 3:
+        raise RuntimeError(
+            f'OpenCV >=3 is required but {cv2.__version__} is installed')
+except ImportError:
+    # If first not installed install second package
+    CHOOSE_INSTALL_REQUIRES = [('opencv-python-headless>=3',
+                                'opencv-python>=3')]
+    for main, secondary in CHOOSE_INSTALL_REQUIRES:
+        install_requires.append(choose_requirement(main, secondary))
 
 def readme():
     with open('README.rst', encoding='utf-8') as f: