Japan

A white rectangular flag with a crimson disc at the center.

Construction

$$\begin{aligned}\text{─}\!\cap\!\text{─} &\times 5\\\text{─}\!\cap\!\bigcirc &\times 13\\\bigcirc\!\cap\!\bigcirc &\times 6\end{aligned}$$

Act on National Flag and Anthem, Act #127, 1999 (2:3 proportion, Disc height, Description, White (implied), Crimson (implied))