A spectrophotometer for measuring a transmission characteristic of a liquid sample, including: an optical system for vertically forming a transmission path of a measurement light in an open space; and a sample holding unit placed in said open space, which is inserted in the transmission path of a measurement light formed by the optical system, for holding a liquid sample, wherein the sample holding unit comprises: a) pedestal unit composed of base plate and a sample stage being a light-transmitting body which is made of a light-transmitting material and whose upper surface and lower surface are both horizontal, the upper surface of the light-transmitting body projecting upwards with respect to an upper surface of the base plate; and b) a window plate which is made of a light-transmitting material and is adapted to be held above the light-transmitting body and on the liquid sample dropped onto the upper surface of the light-transmitting body, in such a manner that a boundary phase parallel to the upper surface of the light-transmitting body is formed by a lower surface of the window plate at a predetermined distance from the upper surface of the light-transmitting body, wherein the spectrophotometer is thereby adapted to allow a measurement of a transmission characteristic of a liquid sample to be performed by: bringing the lower surface of the window plate and the upper surface of the light-transmitting body closer to each other to reach the predetermined distance after the liquid sample is dropped onto the upper surface of the light-transmitting body; and with the liquid sample touching the upper surface of the light-transmitting body and the lower surface of the window plate, delivering a measurement light from above or below the liquid sample and measuring a light which has passed through the liquid sample downwards or upwards, wherein the light-transmitting body and the window plate, or a member holding each of the light-transmitting body and the window plate is made to touch a regulatory member disposed in a space between the light-transmitting body and the window plate in order to set a distance between the upper surface of the light-transmitting body and the lower surface of the window plate to be the predetermined distance. a) pedestal unit composed of base plate and a sample stage being a light-transmitting body which is made of a light-transmitting material and whose upper surface and lower surface are both horizontal, the upper surface of the light-transmitting body projecting upwards with respect to an upper surface of the base plate; and b) a window plate which is made of a light-transmitting material and is adapted to be held above the light-transmitting body and on the liquid sample dropped onto the upper surface of the light-transmitting body, in such a manner that a boundary phase parallel to the upper surface of the light-transmitting body is formed by a lower surface of the window plate at a predetermined distance from the upper surface of the light-transmitting body, bringing the lower surface of the window plate and the upper surface of the light-transmitting body closer to each other to reach the predetermined distance after the liquid sample is dropped onto the upper surface of the light-transmitting body; and with the liquid sample touching the upper surface of the light-transmitting body and the lower surface of the window plate, delivering a measurement light from above or below the liquid sample and measuring a light which has passed through the liquid sample downwards or upwards, wherein the light-transmitting body and the window plate, or a member holding each of the light-transmitting body and the window plate is made to touch a regulatory member disposed in a space between the light-transmitting body and the window plate in order to set a distance between the upper surface of the light-transmitting body and the lower surface of the window plate to be the predetermined distance.