PURPOSE: To provide a pulsation reducing device which saves the manpower and performs rapid and accurate regulation by a method wherein when a fluctuation in a liquid pressure exceeds a specified range during suppression of pulsation phenomenon of pump transfer liquid by a reciprocating pump, a gas-sealing pressure is automatically regulatable according to the situation.

CONSTITUTION: When a diaphragm 14 is moved from a reference position S in a direction, in which a liquid chamber is expanded, owing to a fluctuation in a reduction pressure in a liquid chamber 15, when a stroke exceeds a set value A, a valve 25 is opened through a valve push rod 28 by means of the diaphragm 14, and automatic regulation is effected such that a gas pressure is further exerted on the interior of an air chamber 16 and a sealing pressure is increased. When the diaphragm 14 is moved from the reference position S in a direction in which the liquid chamber is reduced, when a stroke exceeds a set value B, automatic regulation is effected so that a valve 34 is moved to an opening position by means of a slider 33, a gas pressure in the air chamber 16 is released and a sealing pressure is decreased.