A battery pack cooling system in which cell modules (31, 32, 33) configured from a plurality of cells are provided in an internal space of a battery pack case, thereby cooling the cell modules (31, 32, 33) by cooling air flowing in a cooling passage that is secured between the cells that are mutually adjacent, wherein: the cooling passage is configured comprising a cooling air inlet passage, a cooling air exhaust passage, and a plurality of cooling branch passages (61, 62, 63) disposed to link the cooling air inlet passage and the cooling air exhaust passage in parallel, the cell modules (31, 32, 33) are provided respectively in the plurality of cooling branch passages (61, 62, 63), a minimum temperature sensor is installed in an upstream position of one cell module (31, 32, 33), the upstream position being a position which becomes the lowest temperature, and a maximum temperature sensor is installed in a downstream position of the one cell module (31, 32, 33), the downstream position being a position which becomes the highest temperature, in one cell module (31, 32, 33) among the plurality of cell modules (31, 32, 33), a temperature sensor (11h, 13L) that is configured to measure the temperature of either the highest temperature or the lowest temperature is installed in one of the other cell modules (31, 32, 33), among the plurality of cell modules (31, 32, 33), a controller that is configured to carry out a calculation process based on temperature information from the temperature sensors (11h, 12L, 12h, 13L) is provided, and the controller comprises a diagnostic unit that is configured to carry out rationality diagnosis of the temperature sensors (11h, 12L, 12h, 13L) using a difference between temperature change gradients from two minimum temperature sensors (12L,13L) in which the minimum temperatures can be compared, or a difference between temperature change gradients from two maximum temperature sensors (11h, 12h) in which the maximum temperatures can be compared. the cooling passage is configured comprising a cooling air inlet passage, a cooling air exhaust passage, and a plurality of cooling branch passages (61, 62, 63) disposed to link the cooling air inlet passage and the cooling air exhaust passage in parallel, the cell modules (31, 32, 33) are provided respectively in the plurality of cooling branch passages (61, 62, 63), a minimum temperature sensor is installed in an upstream position of one cell module (31, 32, 33), the upstream position being a position which becomes the lowest temperature, and a maximum temperature sensor is installed in a downstream position of the one cell module (31, 32, 33), the downstream position being a position which becomes the highest temperature, in one cell module (31, 32, 33) among the plurality of cell modules (31, 32, 33), a temperature sensor (11h, 13L) that is configured to measure the temperature of either the highest temperature or the lowest temperature is installed in one of the other cell modules (31, 32, 33), among the plurality of cell modules (31, 32, 33), a controller that is configured to carry out a calculation process based on temperature information from the temperature sensors (11h, 12L, 12h, 13L) is provided, and the controller comprises a diagnostic unit that is configured to carry out rationality diagnosis of the temperature sensors (11h, 12L, 12h, 13L) using a difference between temperature change gradients from two minimum temperature sensors (12L,13L) in which the minimum temperatures can be compared, or a difference between temperature change gradients from two maximum temperature sensors (11h, 12h) in which the maximum temperatures can be compared.