To prevent sea casuality caused by capsizing by providing a roll cycle counting part, inputting a reference roll cycle for a chip beforehand, calculating the lower limit values of a caution area and a danger area, making arithmetic comparison to determine if a roll cycle exceeds the lower limit values and outputting a caution or danger signal.

A roll cycle counting part 1 counts a roll cycle Tr, turns ON/ OFF the circuit of a top switch for displaying on a clock part 2, and a measured value is inputted to a calculation part 3, arithmetic-compared with the lower limit value TA of a caution area or the value of a danger area TD stored in a memory by calculation performed based on a reference roll cycle Ts inputted beforehand, and if a roll cycle Tr exceeds the lower limit value TA of the caution area, a caution signal is transmitted to an alarm part 4, and alternatively if the cycle TA exceeds the lower limit TD of the danger area, a danger signal is transmitted to the same. In the alarm part 4, if a caution signal is inputted from the calculation part 3, an alarm sound is produced by turning ON/OFF the red lamp of the display plate of the clock part 2 and, if a danger signal is inputted, by continuously lighting the red lamp, a sharp danger sound is issued.