A knee joint stimulation device comprising: a base; a frame provided upright on the base; a center shaft provided at an upper part of the frame; a crank connecting a motor and the center shaft and driven by rotation of the motor, the center shaft being pivotable via the crank; a swing arm hanging down perpendicularly from the center shaft or a pair of swing arms hanging down perpendicularly from both sides of the center shaft across the motor, a length of the swing arm or the swing arms being adjustable; and a support plate unit provided at a lower end of the swing arm or a pair of support plate units provided at lower ends of the pair of swing arms and allowing a user in a sitting position to put a sole or soles of the user thereon, wherein, with the user putting the sole or the soles on the support plate unit or the support plate units, the length of the swing arm or of the swing arms is adjusted such that a pivot point of a knee joint or pivot points of knee joints of the user is or are aligned with the center shaft, wherein, by swinging movement of the swing arm or swing arms, the support plate unit or the support plate units can move the sole or soles resting thereon in a flexion direction or an extension direction about the knee joint or knee joints, wherein the support plate unit is pivotable about a support shaft or the support plate units are pivotable about support shafts that horizontally extends or extend from the lower end or ends of the swing arm or swing arms, and wherein, even when the sole or soles resting on the support plate unit or units moves or move in a flexion direction or an extension direction about the knee joint or the knee joints and thereby the ankle joint is caused or the ankle joints are caused to move in a plantarflexion direction and a dorsiflexion direction, the support plate unit or the support plate units can pivotally follow the movement of the sole or the soles. a base; a frame provided upright on the base; a center shaft provided at an upper part of the frame; a crank connecting a motor and the center shaft and driven by rotation of the motor, the center shaft being pivotable via the crank; a swing arm hanging down perpendicularly from the center shaft or a pair of swing arms hanging down perpendicularly from both sides of the center shaft across the motor, a length of the swing arm or the swing arms being adjustable; and a support plate unit provided at a lower end of the swing arm or a pair of support plate units provided at lower ends of the pair of swing arms and allowing a user in a sitting position to put a sole or soles of the user thereon, wherein, with the user putting the sole or the soles on the support plate unit or the support plate units, the length of the swing arm or of the swing arms is adjusted such that a pivot point of a knee joint or pivot points of knee joints of the user is or are aligned with the center shaft, wherein, by swinging movement of the swing arm or swing arms, the support plate unit or the support plate units can move the sole or soles resting thereon in a flexion direction or an extension direction about the knee joint or knee joints, wherein the support plate unit is pivotable about a support shaft or the support plate units are pivotable about support shafts that horizontally extends or extend from the lower end or ends of the swing arm or swing arms, and wherein, even when the sole or soles resting on the support plate unit or units moves or move in a flexion direction or an extension direction about the knee joint or the knee joints and thereby the ankle joint is caused or the ankle joints are caused to move in a plantarflexion direction and a dorsiflexion direction, the support plate unit or the support plate units can pivotally follow the movement of the sole or the soles.