The invention discloses a device for measuring the concentration of alcohol contained in the dampening water used in printing. Part of the dampening water pumped up from a dampening water tank (1) and delivered to a water fountain (3) is sampled through a bypass passage and the dampening water is returned to the dampening water tank (1) after the measurement of the concentration of the sampled dampening water. A measuring unit (8) for detecting the concentration of alcohol has a shielding plate (13) disposed adjacent to the dampening water inlet so that the disturbance of the surface of the dampening water introduced into the measuring unit is prevented. The dampening water discharge outlet consists of a pipe whose upper end is in coplanar relationship with a predetermined surface level of the aqueous solution in the measuring unit so that the excessive dampening water overflows into the pipe (10) to return to the dampening water tank. The measuring unit (8) includes a temperature sensor (15), a gas sensor (16) and a pH sensor (50) when required and the gas sensor (16) is disposed within a predetermined closed space.