Muscle tone, an after-effect of stroke, inhibits free limb movement causing muscle shortening, contractures, and joint alterations. A method to reduce muscle tone is manual therapy, called the “piston finger technique”. We previously developed a piston device for fingers, named “PDFin”, that imitates the manual technique, and has applied the device to patients with stroke-related hemiplegia with certain success. In this study, we conducted intervention using the PDFin in eight stroke survivors for 30 s and 10 min and verified the immediate and sustained effects of muscle tone reduction using the modified Ashworth scale (MAS). After each intervention, participants performed a 6-meter walk (WA) and MAS test five times at one-minute intervals. The results showed that immediate effect of the 10-minute intervention on MAS score improvement was significantly greater than that of the 30-second intervention. Moreover, there was a significant difference between the 30-second and 10-minute interventions in the sustained effect on MAS score improvement after the intervention. In particular, the MAS score after the 10-minute intervention and five WA tests did not decrease to the MAS score before the 10-minute intervention in all participants. However, the MAS score after the 30-second intervention and five WAs decreased to the MAS score before the 30-second intervention in 5 participants. The findings showed that a 10-minute intervention using the PDFin further reduced muscle tone, and the effect lasted longer.