A viscosity measurement device (100) mounted in a vehicle (1) equipped with: an engine (10); a cylinder pressure sensor (24) that detects the cylinder pressure, that is, the cylinder pressure of the engine; a fuel injection valve (19) that supplies fuel to the engine; and a temperature sensor (23) that detects the temperature of an engine cooling liquid. This viscosity measurement device is equipped with an estimation means (21) that calculates the cooling loss from a heat generation amount pertaining to the cylinders, said amount being based on the cylinder pressure detected by the cylinder pressure sensor, and from a heat supply amount pertaining to the cylinders, and this estimation means estimates the viscosity of the cooling liquid on the basis of the calculated cooling loss.