Iroha-zaka winding road is a main access to connect central Nikko and Oku-Nikko. You will use the Second Iroha-zaka to go up, and will use the First Iroha-zaka to come down. Each corner has a letter of ancient Japanese alphabet, and you will see it in alphabetical order. The alphabetical order starts from I-ro-ha, while modern alphabet starts from a-i-u.

This road used to be for ascetics in the past. Women and horses were not allowed to go up the slope, and entrance of the slope was called Umagaeshi, which meant returning horse. Also, Nyonin-doh women' s hall is located on the place where women returned after they worshipped Mt. Nantai from a long distance. In addition, trace of Nakano-chaya rest house is located in between Umagaeshi and Nyonin-doh hall. Moreover, giant stone, which has a magnetic force remains there.

People started calling the slope Iroha-zaka in the early Showa era. Ancient Japanese alphabet consisted of 48 letters, and the number of curve is 48. Therefore, tourist guides started calling the slope Iroha-zaka. Today, the number of curve is 30 because the road was improved in 1954. Today's First Iroha-zaka was made in that time. In addition, the Second Iroha-zaka (Up only) was made to adjust more traffic. The number of curves increased up to 50, but 2 curves were decreased in order to match to the 48 letters.