A concentration degree determination device includes: a monitoring data acquisition unit configured to acquire monitoring data from a sensor that monitors a driver; a concentration degree estimator configured to estimate a driving concentration degree of the driver from the monitoring data; a reference comparator configured to compare the driving concentration degree to a first-road reference when a vehicle is running on a first road, and to compare the driving concentration degree to a second-road reference when the vehicle is running on a second road; and a signal output unit configured to output an instruction signal issuing an instruction to perform support to the driver after elapse of a first time when the driving concentration degree does not satisfy the first-road reference, and to output the instruction signal after elapse of a second time shorter than the first time when the driving concentration degree does not satisfy the second-road reference.