Abstract: A physiological information processing device includes one or more processors, and one or more memories for storing computer-readable commands. When the computer-readable commands are executed by the processors, the physiological information processing device executes processing of acquiring attribute information associated with an attribution of an infant, acquiring state information related to a state of the infant before a start of a pain event that causes pain in the infant, acquiring physiological information data of the infant, acquiring facial expression information associated with infant's facial expression after the start of the pain event, calculating a pain assessment index for assessing the pain of the infant in the pain event based on the attribute information, the state information, the physiological information data, and the facial expression information, and outputting the calculated pain assessment index.