The aim of this study was to investigate the relationship between sleep of healthy infants and their mother’s parenting stress in Japan. Data obtained from 48 healthy mothers and their infants aged from 6-month old to 24-month old were studied. The mothers completed questionnaires including the Parenting Stress Index Short Form (PSI Short Form), and infant’s sleep measures were assessed by an actigraph (Micro-mini RC, Ambulatory Monitoring Inc., Ardsley, NY). All infants, except one, shared a room with their mothers for sleeping. Of these infants, 22 infants shared a bed with their mothers. Sleep measures including sleep quality showed developmental change along with their age. However, the score of maternal parenting stress did not change. Maternal parenting stress is significantly differed by the mother’s perception of infant’s sleep. Room-share is common lifestyle among Japanese mothers raising infants and sleep development of infants was confirmed. Thus, the health care providers could give information that bed-share and room-share does not affect infant’s sleep development during early stage of infancy in Japan. Maternal subjective perception of infant’s sleep is associated with parenting stress rather than sleep assessments by objective methods.