The Harjukoski water mill is located in the Ihastjärvi village in Mikkeli. In addition to the mill, there is a mill cottage, two granaries, a pig house, and a mill sauna, where those waiting for milling could stay overnight. On the sauna stove, they fried pork and made porridge. They passed the time doing handicrafts or taking care of the horses. Originally located further away, the smithy was moved near the courtyard in 2006.

All the buildings in the mill area are unclad log constructions with a shingle roof. The stones of the double-burrstone mill are natural stones, and the mill machinery is all made of timber. The mill has kept is original look and feel, and due to its construction engineering, it is of historical value. The water mill and its surroundings are protected in the regional land use plan.

The first application to found the Harjukoski mill was made in 1790, and the mill was decreed to pay tax as of 1841. After 1865, the mill was demolished when the Harjumaa peasants complained that the floodwater covered their waterfront meadows. The current mill, located right on the shore of Kyyvesi, was built to order for Daniel Nykänen, a farm-owner, in the 1870s as a common mill for the village. Later the mill changed owners until 1912, when it became the property of Israel Lappalainen, whose family then ran the mill until it stopped its operation. Starting from the 1930s, different machine powered mills spread to Ihastjärvi, and since 11 farms had free grinding rights, the last miller, Aapeli Lappalainen, ended the operation as unprofitable in 1975 and donated the mill and its buildings to the Mikkeli rural municipality. The mill still serves as a gathering place for the villagers, and they also take care of the maintenance of the area.

The film "The Howling Miller" based on Arto Paasilinna's novel was filmed in the remarkable landscape unit of the Harjukoski mill in 1982. The leading role was played by Vesa-Matti Loiri.