A deterioration estimation device includes a storage device and an execution device. The storage device is configured to store mapping data defining a mapping that outputs an output variable indicating the degree of deterioration of oil when an input variable is input. The mapping includes, as the input variable, a color variable that is a variable indicating a color of the oil and a hydrogen ion variable that is a variable indicating a hydrogen ion concentration of the oil. The execution device is configured to execute an acquisition process that is a process of acquiring the input variable and a calculation process of inputting the input variable acquired through the acquisition process to the mapping to output a value of the output variable.