A two-port hydraulic valve comprising: a housing (10; 110) including first and second ports (111, 112), a first hydraulic chamber (13A; 113A), and a second hydraulic chamber (13B; 113B), wherein the first and second hydraulic chambers (13A; 113A, 13B; 113B) are communicated with the first and second ports (111, 112), respectively; a valve seat (40; 130) located in the first hydraulic chamber (13A; 113A), wherein the valve seat (40; 130) has a tubular shape including a central passage and an open end, wherein the open end includes a valve seat contact surface (46, 47); a valve member (51; 80; 141) located in the second hydraulic chamber (13B; 113B) and including a valve member contact surface; and a biasing member (31, 32; 160) that generates a biasing force to press the valve seat (40; 130) and the valve member (51; 80; 141) so that the valve member contact surface comes in planar contact with the valve seat contact surface (46, 47) which is flat, wherein when hydraulic pressure of the first hydraulic chamber (13A; 113A) is higher than that of the second hydraulic chamber (13B; 113B), the hydraulic pressure of the first hydraulic chamber (13A; 113A) acts on the valve member (51; 80; 141) through the central passage of the valve seat (40; 130) to separate the valve member (51; 80; 141) from the valve seat (40; 130) against the biasing force of the biasing member (31, 32; 160) so that the first hydraulic chamber (13A; 113A) and the second hydraulic chamber (13B; 113B) come into fluidal communication, and when hydraulic pressure of the second hydraulic chamber (13B; 113B) is higher than that of the first hydraulic chamber (13A; 113A), a hydraulic pressure difference between the first hydraulic chamber (13A; 113A) and the second hydraulic chamber (13B; 113B) acts on the valve seat (40; 130) to separate the valve seat (40; 130) from the valve member (51; 80; 141) against the biasing force of the biasing member (31, 32; 160) so that the first hydraulic chamber (13A; 113A) and the second hydraulic chamber (13B; 113B) come into fluidal communication; wherein the biasing member (31, 32; 160) includes a first spring that presses the valve seat (40; 130) toward the valve member (51; 80; 141), and a second spring opposed to the first spring, wherein the second spring presses the valve member (51; 80; 141) toward the valve seat (40; 130); and wherein the hydraulic valve further comprises an auxiliary oil channel that is in communication with the first hydraulic chamber so that the hydraulic pressure of the first hydraulic chamber assists the valve member (51; 80; 141) when the valve member (51; 80; 141) presses the valve seat (40; 130). a housing (10; 110) including first and second ports (111, 112), a first hydraulic chamber (13A; 113A), and a second hydraulic chamber (13B; 113B), wherein the first and second hydraulic chambers (13A; 113A, 13B; 113B) are communicated with the first and second ports (111, 112), respectively; a valve seat (40; 130) located in the first hydraulic chamber (13A; 113A), wherein the valve seat (40; 130) has a tubular shape including a central passage and an open end, wherein the open end includes a valve seat contact surface (46, 47); a valve member (51; 80; 141) located in the second hydraulic chamber (13B; 113B) and including a valve member contact surface; and a biasing member (31, 32; 160) that generates a biasing force to press the valve seat (40; 130) and the valve member (51; 80; 141) so that the valve member contact surface comes in planar contact with the valve seat contact surface (46, 47) which is flat, wherein when hydraulic pressure of the first hydraulic chamber (13A; 113A) is higher than that of the second hydraulic chamber (13B; 113B), the hydraulic pressure of the first hydraulic chamber (13A; 113A) acts on the valve member (51; 80; 141) through the central passage of the valve seat (40; 130) to separate the valve member (51; 80; 141) from the valve seat (40; 130) against the biasing force of the biasing member (31, 32; 160) so that the first hydraulic chamber (13A; 113A) and the second hydraulic chamber (13B; 113B) come into fluidal communication, and when hydraulic pressure of the second hydraulic chamber (13B; 113B) is higher than that of the first hydraulic chamber (13A; 113A), a hydraulic pressure difference between the first hydraulic chamber (13A; 113A) and the second hydraulic chamber (13B; 113B) acts on the valve seat (40; 130) to separate the valve seat (40; 130) from the valve member (51; 80; 141) against the biasing force of the biasing member (31, 32; 160) so that the first hydraulic chamber (13A; 113A) and the second hydraulic chamber (13B; 113B) come into fluidal communication; wherein the biasing member (31, 32; 160) includes a first spring that presses the valve seat (40; 130) toward the valve member (51; 80; 141), and a second spring opposed to the first spring, wherein the second spring presses the valve member (51; 80; 141) toward the valve seat (40; 130); and wherein the hydraulic valve further comprises an auxiliary oil channel that is in communication with the first hydraulic chamber so that the hydraulic pressure of the first hydraulic chamber assists the valve member (51; 80; 141) when the valve member (51; 80; 141) presses the valve seat (40; 130).