A threaded connection connecting a pair of pipes includes a pin and a box. The pin has a male thread on its outer diameter. The box has a female thread on its inner diameter, the female thread corresponding to the male thread. The male thread includes constant-thread-width and varying-thread-width portions. The constant-thread-width portion has a constant thread-groove width. The varying-thread-width portion has a thread-groove width equal to or larger than the groove width of the constant-thread-width portion and gradually increasing, from the constant-thread-width portion toward the tip of the pin. The female thread includes constant-thread-width and varying-thread-width portions. The constant-thread-width portion has a constant thread-ridge width. The varying-thread-width portion has a thread-ridge width equal to or larger than the thread-ridge width of the constant-thread-width portion and gradually increasing from the constant-thread-width portion toward the center of the box.