An operation state monitoring device comprising a fluid supply source , a first and second tube, and a cylinder in which a piston connected to a piston rod is reciprocally moved between one end and another end inside a cylinder body , by a first cylinder chamber being formed between the one end and the piston inside the cylinder body , together with a second cylinder chamber being formed between the other end and the piston inside the cylinder body , and by a fluid being supplied from the fluid supply source to the first cylinder chamber via the first tube , or the fluid being supplied from the fluid supply source to the second cylinder chamber via the second tube , the operation state monitoring device further comprising: a first pressure detecting unit configured to detect a pressure (P1) of the fluid inside the first tube ; a second pressure detecting unit configured to detect a pressure (P2) of the fluid inside the second tube ; and a determination unit configured to determine whether or not the piston has reached the one end or the other end inside the cylinder body , on a basis of the respective pressures (P1, P2) detected by the first pressure detecting unit and the second pressure detecting unit , wherein the determination unit is configured to determine whether or not the piston has reached the one end or the other end inside the cylinder body , on a basis of a differential pressure (ΔP12, ΔP21) between a first pressure value (P1) which is a pressure value of the fluid inside the first tube detected by the first pressure detecting unit , and a second pressure value (P2) which is a pressure value of the fluid inside the second tube detected by the second pressure detecting unit , and characterized in that on a basis of the differential pressure (ΔP12, ΔP21) between the first pressure value (P1) and the second pressure value (P2), and a sign of the differential pressure (ΔP12, ΔP21), the determination unit is configured to determine which of the one end and the other end inside the cylinder body that the piston has reached. a first pressure detecting unit configured to detect a pressure (P1) of the fluid inside the first tube ; a second pressure detecting unit configured to detect a pressure (P2) of the fluid inside the second tube ; and a determination unit configured to determine whether or not the piston has reached the one end or the other end inside the cylinder body , on a basis of the respective pressures (P1, P2) detected by the first pressure detecting unit and the second pressure detecting unit , wherein the determination unit is configured to determine whether or not the piston has reached the one end or the other end inside the cylinder body , on a basis of a differential pressure (ΔP12, ΔP21) between a first pressure value (P1) which is a pressure value of the fluid inside the first tube detected by the first pressure detecting unit , and a second pressure value (P2) which is a pressure value of the fluid inside the second tube detected by the second pressure detecting unit , and characterized in that on a basis of the differential pressure (ΔP12, ΔP21) between the first pressure value (P1) and the second pressure value (P2), and a sign of the differential pressure (ΔP12, ΔP21), the determination unit is configured to determine which of the one end and the other end inside the cylinder body that the piston has reached. <EOS>