From Sapporo (how to get to Sapporo):
From December to early April, multiple buses per day, including the Good Sports White Liner, Donan buses (which continue on to Niseko or Lake Toya) and Hokkaido Access Network buses, operate between Sapporo and Rusutsu Resort.
The one way trip takes two hours and costs approximately 2000 yen. Reservations are required. Buses leave from JR Sapporo Station and several major hotels in the city. Packages including lift tickets and round trip bus tickets are available.
Outside of winter, only Donan Bus operates seven round trips per day between Sapporo and Lake Toya, which stop at Rusutsu along the way.
From New Chitose Airport:
Multiple buses per day, including the Good Sports White Liner, the Donan Bus White Dolphin (which continues on to Niseko) and Hokkaido Access Network buses, operate between Sapporo's New Chitose Airport and Rusutsu. The one way trip takes about two hours and cost approximately 2000 yen. Reservations are required. Packages including lift tickets and round trip bus tickets are available.
Most of the above mentioned buses operate from December to early April. There are no buses outside of winter.
From Lake Toya (Toyako):
Donan buses between Lake Toya and Sapporo stop in Rusutsu along the way. The one way trip between Lake Toya and Rusutsu takes 45 minutes and costs 1120 yen. There are seven buses per day throughout the year, and reservations are required.