This publication discloses an apparatus and method for measuring a gas content. The apparatus includes a light transmitter (1), by means of which coherent light can be sent to the measurement object (3), a receiver (2), by means of which light that has passed through the measurement object (3) can be detected, and optical means (4, 5, 6), by means of which the light intensity of the light transmitter (1) can be aimed at the receiver (2). According to the invention, the optical means include a lens element (4), the optical axis of which is arranged essentially obliquely relative to the longitudinal axis of the measurement object (3), so that the angles of the normals of the optical boundaries relative to the measuring signal are set obliquely. The lens element (4) is both a refracting and a reflecting element, and separates the measurement object (3) from the means (1, 2, 5).