Abstract: A pressure checker for a fluid pressure source, including a main unit which includes/comprises a hollow portion which can communicate with a fluid pressure source, a cap with a window fitted on the main unit, a spool contained in the hollow portion of the main unit to be reciprocal, said spool being provided with a front end portion with a defined pressure indication surface, a compression coil spring for biasing the spool against the pressure of the fluid pressure source, and a flat light-shielding member positioned in the window of the cap, said flat light-shielding member being provided with a bag made of a material which allows at least partial transmission of light, and a light-absorbing liquid sealed in the bag, wherein the flat light-shielding member transmits light through the window toward the defined pressure indication surface when the spool is moved by the pressure of the fluid pressure source against the biasing force of the compression coil spring, and can also transmit light reflected by the de