diff --git a/jsk_pr2_robot/jsk_pr2_accessories/pr2_hand_apriltag/README.md b/jsk_pr2_robot/jsk_pr2_accessories/pr2_hand_apriltag/README.md
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+## Motivation
+PR2's calibrations (intrinsics/extrinsics/kinematics) are very challenging.
+A simple but effective workaround is to attach markers to the PR2's gripper and compare the pose calculated by nominal kinematics with the detected pose.
+We have been using yellow tape as markers, but it's difficult to maintain their position and orientation consistently.
+Therefore, we decided to create a 3D-printed tag plate for the PR2's gripper.
+
+## Requirements
+Software:
+```bash
+sudo apt install texlive-base openscad
+```
+Hardware:
+- スペーサM3オスメス六角10mm (x2) (Recommended to use plastic spacers to not damage the aluminum lid)
+- 六角穴付きボルトM3-10mm (x2)
+
+## Assebly instructions
+- Then run `make` command in this directory.
+- Print the generated PDF file under `build/april_tag_print.pdf`
+- 3D print the generated STL file under `build/tag_plate.stl`
+- Remove the two bolts from the lid of PR2's gripper's back side. Don't remove the lid.
+- Attach the printed tag plate to the lid using the two bolts with the spacers.
+- Paste the printed tag on the tag plate. The printed tag size is supposed to be exactly equal to the tag plate size.
+
+See the images under [./docs_image/](docs_image/) for the reference.
+
+## Example usage
+- launch `example/kinect_april_tag_detection.launch` inside PR2's machine.
+- Check tf tree to see `april` frame.
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