Objective: We quantitatively evaluated the gait of Parkinson’s disease (PD) patients over a 10-m course during normal walking and during dual-task walking while performing a calculation task, and clarified which parts of white matter lesions (WML) influence gait in PD patients. Methods: Gait parameters, including walking speed, gait cycle, stride length, and left-right instability, were measured in 64 PD patients and 20 controls who walked 10 m with normal gait and as they were performing a calculation task. WML on magnetic resonance imaging (MRI) of PD patients were scored according to Scheltens’ criteria, and associations with gait parameters were investigated. Results: Compared to controls, the PD group showed decreased walking speed and narrowed stride ( p < 0.05), and the stride length and step time coefficient of variation changed significantly during the calculation task ( p < 0.001). Frontal lobe functions correlated positively with walking speed and stride during the calculation task in patients with PD ( p < 0.05). The total score for periventricular hyperintensity (PVH) on MRI correlated with walking speed and stride ( p < 0.01). Multiple regression analysis revealed significant correlations between walking speed and frontal cap of PVH, and between stride and occipital cap ( p < 0.05). Conclusion: Gait of PD patients deteriorated not only due to motor dysfunction but also due to mental burden in association with frontal lobe function and periventricular lesions of cerebral white matter.