A component measuring device for measuring the quantity and/or the property of a given component in a specimen by calorimetrically measuring a test strip includes a tip mount on which is to be mounted a tip having the test strip, and a photometric unit. The photometric unit comprises a light-emitting element that applies light to the test strip while the tip is mounted on the tip mount, a light-detecting element that detects light reflected from the test strip, and a holder in which is held the light-emitting element and the light-detecting element. The holder possesses a passage through which passes the light and the reflected light. A light-transmissive member closes the passage with a sealing member interposed therebetween in a portion of the holder which faces the test strip. The component measuring device can also be provided with a stain detecting device which is adapted to detect a stain on the light-transmissive member.