To solve the problem that the blockage of a connecting pipe cannot be detected accurately since rise or fall components appear while being rocked when pressure and differential pressure are rising or falling.

Three pressure or differential pressure measurement values that are continuous in time are used to compute rocking by the following expression. Since the influence cannot appear in rocking contents even if the pressure or differential pressure varies, the blockage of the connecting pipe can be detected accurately. In rocking=P(i)-2×P(i-1)+P(i-2), P(i), P(i-1), and P(i-2) indicate pressure or differential pressure at time (i), i-1, and i-2, respectively.