A pressure tank having a simple structure to intermittently feed powder to a conveying pipe, and a method for determining the intervals for feeding the powder to the conveying pipe. The pressure tank includes a tank body used as a structure of a pressure vessel having a receiving port at its upper portion and a discharging port at its lower portion, a first check valve disposed just under the receiving port so that it can freely move up and down, and so that it can close the receiving port by its upward movement that is caused by means of compressed air, and a second check valve disposed just under the discharging port so that it can freely move up and down, and so that it can close the discharging port by its upward movement that is caused by means of compressed air. To determine the intervals for feeding the powder to the conveying pipe, the apparatus is controlled based on measurements of the pressure loss or of the time that is required for feeding the powder to the conveying pipe, so that the pressure loss of the compressed air in the conveying pipe corresponds to the pressure of the compressed air supplied to the conveying pipe.