Recently, a simple device for self-monitoring of daily salt intake was developed, and it is recommended by The Japanese Society of Hypertension. This study aimed to investigate the effects of this device on salt reduction and on lowering blood pressure. In this single blinded, cluster randomized controlled trial, families were randomly assigned to either an intervention or a control group. Participants in both groups attended lectures about salt reduction, but only the intervention group used the self-monitoring device to estimate their daily salt intake. The main outcome measure was the difference in the estimated daily salt intake by spot urine between the two groups after 4 weeks. The secondary outcome was the difference in blood pressure. A total of 105 families (158 participants) were randomized. The mean daily salt intake was 9.04 (SD 1.77) g/day in the control group and 9.37 (SD 2.13) g/day in the intervention group at baseline. After 4 weeks, the mean daily salt intake was 8.97 (SD 1.97) g/day in the control group and 8.60 (SD 2.25) g/day in the intervention group; the mean difference between the two groups was -0.50 g/day (95% confidence interval (CI) -0.95, -0.05; P = 0.030). The mean difference in systolic blood pressure was -4.4 mm Hg (95% CI -8.7, -0.1; P = 0.044). This is the first randomized controlled trial to demonstrate the effectiveness of a device for self-monitoring of salt intake with a significant reduction in daily salt intake and systolic blood pressure.