A pressure sensor comprising: a Si device for detecting a pressure P, which is an object of detection, through detection of a stress occurring to the Si device itself by the pressure P, the Si device having a rectangular plate shape, formed from a plate-shaped SOI substrate and having (a) a first main surface that has a bonding area, (b) a second main surface that is parallel to the first main surface, (c) a pressure-sensitive resistor which is formed on the first main surface and whose resistance changes according to the pressure P by the piezoresistive effect, and (d) a joining assist pattern that is formed on the first main surface and joined to the pressure-sensitive resistor, and a pressing member made of glass and bonded to the bonding area on the first main surface of the Si device through anodic bonding, and compressing the Si device in a thickness direction of the Si device by pressing the bonding area by a pressing force according to the pressure P, and wherein the pressing member is bonded to the first main surface except for four corner sections (130ah1, 130ah2, 130ah3, 130ah4) of the first main surface with the pressing member reaching up to four outer circumferential edges which define the first main surface, the pressure-sensitive resistor having a first bonded section which is placed within the bonding area of the first main surface and is bonded to the pressing member, and the joining assist pattern having a second bonded section which is made of the same material as the first bonded section of the pressure-sensitive resistor and is placed within the bonding area of the first main surface and is bonded to the pressing member. a Si device for detecting a pressure P, which is an object of detection, through detection of a stress occurring to the Si device itself by the pressure P, the Si device having a rectangular plate shape, formed from a plate-shaped SOI substrate and having (a) a first main surface that has a bonding area, (b) a second main surface that is parallel to the first main surface, (c) a pressure-sensitive resistor which is formed on the first main surface and whose resistance changes according to the pressure P by the piezoresistive effect, and (d) a joining assist pattern that is formed on the first main surface and joined to the pressure-sensitive resistor, and (a) a first main surface that has a bonding area, (b) a second main surface that is parallel to the first main surface, (c) a pressure-sensitive resistor which is formed on the first main surface and whose resistance changes according to the pressure P by the piezoresistive effect, and (d) a joining assist pattern that is formed on the first main surface and joined to the pressure-sensitive resistor, and a pressing member made of glass and bonded to the bonding area on the first main surface of the Si device through anodic bonding, and compressing the Si device in a thickness direction of the Si device by pressing the bonding area by a pressing force according to the pressure P, and wherein the pressing member is bonded to the first main surface except for four corner sections (130ah1, 130ah2, 130ah3, 130ah4) of the first main surface with the pressing member reaching up to four outer circumferential edges which define the first main surface, the pressure-sensitive resistor having a first bonded section which is placed within the bonding area of the first main surface and is bonded to the pressing member, and the joining assist pattern having a second bonded section which is made of the same material as the first bonded section of the pressure-sensitive resistor and is placed within the bonding area of the first main surface and is bonded to the pressing member.