A sensor insertion device (1,51), comprising: a lower case (7,57) that has an upper surface opening (9,59); an upper case (8,58) that is disposed around an outer periphery of the lower case (7,57), covering the lower case from above, and configured to be slidable with respect to the lower case (7,57), and that is configured to be operated in direction of needle insertion by a user when performing a needle insertion operation; a sensor base (4,54) that is disposed inside the lower case (7,57) and that is is configured to hold a sensor (6,56) configured to acquire biological information; a needle holder (26,67) that is disposed above the sensor base (4,54) and configured to be movable up and down, and that is configured to hold a guide needle (20,78) which is configured to be inserted into a patient's body (2,52) in order to guide the sensor (6,56) into the patient's body (2,52); and a needle holder raising mechanism (30A,74) configured to raise the needle holder (26,67) holding the guide needle (20,78) that has been inserted into the patient's body (2,52), the needle holder raising mechanism (30A,74) has: a pinion gear (30,71) configured to raise the needle holder (26,67); a first rack gear (33,72) that is provided on the needle holder (26,67) and is engaged with the pinion gear (30,71); and a second rack gear (34,73) that is provided on the upper case (8,58) and is engaged with the pinion gear (30,71), and as the upper case (8,58) is further operated in direction of needle insertion, the pinion gear (30,71) is configured to raise the needle holder (26,67) by engagement with the second rack gear (34,73). a lower case (7,57) that has an upper surface opening (9,59); an upper case (8,58) that is disposed around an outer periphery of the lower case (7,57), covering the lower case from above, and configured to be slidable with respect to the lower case (7,57), and that is configured to be operated in direction of needle insertion by a user when performing a needle insertion operation; a sensor base (4,54) that is disposed inside the lower case (7,57) and that is is configured to hold a sensor (6,56) configured to acquire biological information; a needle holder (26,67) that is disposed above the sensor base (4,54) and configured to be movable up and down, and that is configured to hold a guide needle (20,78) which is configured to be inserted into a patient's body (2,52) in order to guide the sensor (6,56) into the patient's body (2,52); and a needle holder raising mechanism (30A,74) configured to raise the needle holder (26,67) holding the guide needle (20,78) that has been inserted into the patient's body (2,52), the needle holder raising mechanism (30A,74) has: a pinion gear (30,71) configured to raise the needle holder (26,67); a first rack gear (33,72) that is provided on the needle holder (26,67) and is engaged with the pinion gear (30,71); and a second rack gear (34,73) that is provided on the upper case (8,58) and is engaged with the pinion gear (30,71), and as the upper case (8,58) is further operated in direction of needle insertion, the pinion gear (30,71) is configured to raise the needle holder (26,67) by engagement with the second rack gear (34,73). a pinion gear (30,71) configured to raise the needle holder (26,67); a first rack gear (33,72) that is provided on the needle holder (26,67) and is engaged with the pinion gear (30,71); and a second rack gear (34,73) that is provided on the upper case (8,58) and is engaged with the pinion gear (30,71), and as the upper case (8,58) is further operated in direction of needle insertion, the pinion gear (30,71) is configured to raise the needle holder (26,67) by engagement with the second rack gear (34,73).